The seriousness of upraising is because it is heresy

Rambam, Known as Maimonides, foremost codifier of Jewish law (1138-1204)
Our brothers, be strong and steadfast, and rely on these Biblical verses I have quoted. Do not let government persecution overwhelm you. Do not fear the power of the enemy’s hand over you, and the weakness of our people. For this entire situation is only a test to show your faith and wisdom to the world.
(Letter to Yemen)

Rabbi Shmuel ben Yitzchak Yaffe Ashkenazi of Constantinople (1525-1595)
The tribe of Ephraim left Egypt 30 years too early, basing themselves on their understanding of Abraham’s prophecy. So why is this called not believing in G-d and not trusting in His deliverance (Psalms 78:22)? Why is this transgressing the Oath? It was a mere mistake! The answer is that the mistake was deeper than just a misinterpretation of numbers. They thought that when the foretold time came the Jews would leave Egypt with their own strong hand, without the open intervention of G-d. Thus they did not rely on the deliverance of G-d, but on their own swords. Had they understood that the Exodus would be a miraculous event, they would certainly not have taken the initiative without seeing a miracle to demonstrate that this was G-d’s plan. Even when the real redeemer – Moses – eventually came and claimed that G-d had sent him, the Jews were not allowed to believe in him without seeing a miracle. And they had a tradition that the true redeemer would say certain words (“I have surely remembered you”). Since the children of Ephraim had no such redeemer, they could not have left Egypt relying on G-d’s deliverance. Clearly, they thought that they would succeed in leaving Egypt and conquering Canaan through purely natural means. This is why the Midrash says that “they transgressed the Oath” – the oath that prohibited the Jews from leaving the exile on their own, without G-d’s intervention. And “they transgressed the End” – since there was no sign from G-d, they should have realized that their numerical calculation was wrong.
(Commentary Yefeh Kol on the Midrash Shir Hashirim)

Rabbi Azariah Figo (1579-1647)
Why did Samuel the Prophet (I Samuel 12:17) choose the miracle of rain in the summer – to teach the people that they should not have asked for a king? Really, we must first ask why it was wrong of the Jews to request a king, in light of the fact that appointing a king is a mitzvah, and in light of Jacob’s prophecy, “The scepter shall not depart from Yehudah…” (Genesis 49:10) The Ramban in his commentary to that verse in Genesis provides an answer: that the right time for the rule of tribe of Yehudah – Dovid Hamelech and his house – had not yet arrived. Thus when the people requested a king, they were forcing the hour, and so in place of a king from the chosen tribe of Yehudah, they were given Saul, from the tribe of Binyamin, the last of the tribes. And because Israel trespassed over the border of the nations, so to speak, by appointing a king of their own while the nations’ kings were still ruling, they caused the nations to trespass against them later. Had they waited till the proper time, perhaps no other nation would ever have been able to dominate them.

Since the request for a king would have been proper in the right time, but in the wrong time it was considered a sin and an ominous event, it is exactly comparable to a request for rain in the summer. When rain falls in its proper time, it is a blessing and a good fortune; we always ask for rain in our prayers in that season. But we do not pray for rain in the summer, and if someone does so accidentally he must repeat his prayers, for rain in that season is not desirable. Shmuel said to the people, “Your request is similar to the request I am about to make: I will call out to G-d and He will send rain now, in the summer. Had you waited till the proper time to ask for a king, it would have been fine, but now you have acted foolishly, for you are implying that there is something lacking in the providence of G-d, Who planned for you to have a king later, not now.”
(Binah L’itim עת צאת דרוש כד)

Rabbi Sholom Dov Ber Schneersohn, Lubavitcher Rebbe, Russia (1866-1920)
In order to get Jews to believe in their idea of building our own country, the Zionists of course had to preach nationalism. But this alone was not sufficient; they had to convince Jews to abandon Torah and mitzvos, or at least to weaken their observance as much as possible, making nationalism more important than Torah. For it was obvious that those who keep Torah and mitzvos were not likely to change and accept a different form of nationhood. And especially this model of leaving exile by force, redeeming themselves with their own power – no believing Jew could ever accept, because it goes against their strong belief in the coming of the messiah, who will redeem us physically and spiritually. It was this hope that kept the Jewish soul alive throughout all these years of bitter exile, not Herzl’s promises of an independent state, even if we could fool ourselves into thinking that he could attain one.
(Kuntres Umayan Mibeis Hashem)

Rabbi Yoel Teitelbaum, Satmar Rebbe (1887-1979)
Taking redemption, appointing a ruler and a government before the coming of the messiah is denial of G-d’s providence. And even regarding the tribe of Ephraim who left exile prematurely due to a mere error, due to their calculation that the time had come, the verse in Psalms says, “They did not believe in G-d.” I have already quoted Rabbi Shmuel Yaffe, who says that since it is written in many verses that only G-d will redeem the Jews from exile, doing so on their own goes against faith.
(Vayoel Moshe, Chapter 76)

Rabbi Moshe N. Kahana Shapira, rabbi of Kshonz (1902)
“And he shall surely heal.” (Exodus 21:19) The Talmud (Bava Kama 85a) says, “From here we learn that a doctor is permitted to heal.” Rashi explains that if not for this verse, we might have said, “The Merciful One smites, and this man heals?!” Similarly, Tosafos says that the Torah uses a double expression “heal he shall heal” because if it had only said “heal” one time, we would have thought that a doctor may only heal a wound inflicted by a person, but to heal a sickness would be like going against the decree of the King. Special permission was given for a doctor to heal, but in general the principle remains true: taking action to reverse a situation brought about by G-d is considered rebelling against G-d’s decree.

This is all the more true in the case of effort to end the exile – not only is there no special permission for such effort, but it was explicitly forbidden, as is written all over the Bible. For example, Deuteronomy 30 states: “’And it shall come to pass when all these things come upon you, and you shall take it to your heart among all the nations to which the L-rd your G-d scattered you. You will repent to the L-rd your G-d and listen to His voice, according to all that I command you today, you and your children, with all your hearts and souls. And the L-rd will return your exile and have mercy on you, and He will once again gather you from all the nations… If your exiles are in the edge of heaven, from there the L-rd your G-d will gather you and from there He will take you. And the L-rd your G-d will bring you to the land that your fathers inherited, and you shall inherit it.” We see here that it mentions G-d’s name in connection with the action of ingathering, and repeats it regarding bringing the Jews to their land, to teach that only repentance is our job, as a means to hasten the end of exile, but the act of ingathering and bringing us back are in the hands of G-d alone. As Psalm 127 says, “If G-d does not build a house, its builders toil in vain; if G-d does not guard a city, in vain does the guard watch over it.” And in the Song of Songs 2:7: “I adjure you, daughters of Jerusalem, by the gazelles and deer of the field, not to arouse or awaken the love until it is desired.” But rather, wait for “the voice of My beloved, behold it has come.” And Isaiah (40:1) says: “Comfort, comfort my people, says your G-d. Speak to the heart of Jerusalem… Behold the L-rd your G-d comes strongly,” not in a natural way, but “His arm rules for Him” – with an outstretched arm, i.e in a miraculous way as in the Exodus from Egypt.
(Daas Harabbanim, p. 38)