Speed and ammo selection are kind of a big deal. Speed one of the most valuable attributes a tank can have, it’s what allows you to get into positions to make the gun and/or armor work to their best. Lacking speed means you cannot really do that, you risk being outmanuevered or overwhelmed, and you cannot position aggressively to get a position where the gun is at it’s best.
As far as ammo goes, you have a worse AP round in place of APHE, a worse APDS round (in terms of angled pen and spalling), and no HEAT shell for overpressure. That’s a sizable tradeoff.
And they lose out on a lot of armor. The T-54 is only really weak to high penning AP/APHE in the area immediately around the gun. It’s an area with some volumetric protection, and weaker rounds (for the tier) like the long 88 or American long 90 need very precise shots. Add on to this aggressive hull waggling, and that shot becomes rather difficult, even with rounds that can punch through. The entire hull is resistant to all but the strongest of these rounds. Even the French 100mm can’t punch through if angled.
Meanwhile, the Cent (and Caern) have armor that won’t stop anything stronger than M82 out of the short 90. Even then, the cupola and bottom of the turret will let that in at close ranges. The hull can’t even stop the long 88 at 500m, and even if they are perfectly angled, the LFP is a giant weakspot that anything can get a round through, directly into the ammo rack.
Does the stabilizer and reload speed account for these differences? Hard to say. It’s an apples to oranges comparision, and it depends heavily on playstyle. I’d say not personally.
Haven’t played the T54E2, so someone who has can feel free to correct me, but based on the test drive, it’s slower, gun is similar (AP with slighly more pen versus APHE for much more consistent damage, better APDS, slightly worse HEAT) but with a much slower reload. Armor is so much worse (Vulnerable to M82 in several different spots, most being unhidable). I’d take the T-54 for the speed, armor and reload, better for aggressive play. Though the E2 looks to be better in an uptier
The French are a bizarre comparison, given their complete lack of any reliable armor, radically different gun situation (AP only, autoloader, no options for long range/high armor targets), and entirely different playstyle. Too far removed to be a decent yardstick to measure the T-54s against.