If you're not saying "I should be able to reduce autonomy and this should never bite me in the ass and cause my conquered empire of different cultures to explode", then I really don't understand what you're saying. Empires like that got overthrown by rebellions in real life, you know.
I should be able to reduce autonomy when there is no nationalism or OE anywhere in my nation and I'm not at war and my army is standing by ready to deal with any rebel stacks with basically no risk.
Sure, if I drive my OE over 100 or am engaged in a war and my army is occupied, then rebels should be dangerous.
Can you name three rebellions during the time period which were able to militarily defeat the entire armed strength of the nation they were rebelling against in open battle without foreign support?
Heck, can you name three successful nationalist rebellions during the time period, regardless of circumstance? I can't. I can name one, the Netherlands rebelling against Spain, and it is scripted in EUIV, and succeeded historically because of substantial foreign support (France, Britain, Ottomans), Spain's remoteness from the rebellion, and Spain's financial difficulties, including a bankruptcy in 1575 and a later inability to pay its troops because of the cost of replacing ships after the destruction of the Spanish Armada in 1588 - troops who mutinied over 40 times.
Ie, historically rebels win when they can kick you while you're down. Really down. Rebels always lost when the full armed might of a nation could be brought against them in this time period. And this is exactly how rebels should function in the game. They shouldn't be a military challenge, but a multi-tasking challenge and impose minor to moderate costs in manpower to put down.
I don't want to be rude, but your "guarantee" comes from your ignorance of the game. It's not a crime to be ignorant of game mechanics, but that doesn't mean that just because you think nobody can deal with your situation, that nobody can.
It would help if they'd actually have the game mechanics explained somewhere. I had no idea that a handful of 5% RR provinces would spawn 2.5x my FL of rebels and have a 75% chance of increase revolt progress 5% each month until it happened. I had expected rebels to be tuned so they didn't come in arcade-style numbers and were vaguely historically plausible.
Nor did the tooltip on reduce autonomy say you'd be locked in for 30 years and unable to reverse course. (Talk about ahistorical.)
If I'd known those two things, I'd have been far more paranoid about the reduce autonomy button. (As far as I'm concerned at this point, it's a non-option - the risk in outlier situations is sufficiently extreme to make it never worth using).
Not knowing rules they never bother to tell you on mechanics that just changed in the patch is hardly my own fault. There should be an in-game reference with detailed mechanics explanations.
Your freshly expanded empire, trying to integrate the wrong-culture provinces it conquered, should be possible to be overcome by rebels with bad luck and a bad strategy (both of which you had). You are not meant to win every single time. Lick your wounds, figure out what you did wrong, and try again. Or just start cheating or turning the difficulty down, if you don't wish to do that.
Not freshly expanded. 0 OE. The last province (just one) I conquered was *10 years ago*. The only reason there are nationalist rebels is because they have a different (although accepted) culture - there's no nationalism or religious differences in those provinces at all.
The only thing I did wrong was have my ruler die at younger than 25 with no heir. Unless of course just using the reduce autonomy button is a mistake because your 20-year-old 100 legit ruler could die before he produces an heir even with a substantial heir chance boost (+55% from NI and RMs).