Personal opinion time. I found the SU-101 to be a disappointment in the line. Let's start by comparing it to what you get to drive before and after:
The tier 7, SU-100M1, was pretty wild, with armor that allowed you to effectively be a turretless heavy if matchmaking allowed. It genuinely bullied people, could hull-down with the big boys and get up close and personal. It's listed as an "Assault Tank destroyer" and it truly does just that. The stock SU-100M1 also isn't terrible because the only gun upgrade you get buffs penetration. You get max alpha damage at stock. I averaged 1,611 damage in the dozen of games I played it – not bad.
The Object 263 is something I've also come to enjoy. If you can hide the lower plate with some debris or some face-hugging you're in for a pretty good game. There's only one gun to upgrade, and honestly the stock gun can work because the premium HEAT pen is better (340) than the upgraded gun (300). However, slapping on an extra 110 damage when upgraded is very nice. Your other weakspots are manageable and if you play the 263 on flat ground, you'll do just fine. I'm averaging 2,108 damage and I have not completed the grind on it. Pretty good.
But the SU-101… oh boy. The first red flag is that it's no longer listed as an "Assault Tank Destroyer", it's a "Versatile Tank Destroyer", indicating that it won't accel at anything. And it really doesn't – your armor is about the same as the tier 7 just before it but at a bigger slope. Unfortunately it often feels like this isn't enough, and the armor doesn't bounce nearly as often as the tier 7 and tier 9. The grind has jumped from 2 different guns to 4, and the 3rd gun has the worst dispersion out of them all. It has the worst gun depression. The worst aim time. A lot of the sniper shots were misses, and a lot of the time brawling my armor barely increased my survivability if any. I averaged 1,478 damage. Not great.
The SU-101 seems out of place in the line. The tier 7, 9, and 10 all have good, reliable frontal armor in the front that make them worthy of an Assault Tank Destroyer tech tree. If you're an SU-101 fan, I'm sorry, but the tank is just disappointing.
Source: https://www.reddit.com/r/WorldofTanks/comments/s8jtyu/the_su101_is_a_letdown/