Russians relied heavily on ERA because their composite armor technology was lagging behind the west. Even now they still don't have armor similar to Dorchester or armor installed on eg. M1A2 SEP or Leo 2A6
Actually the opposite is true... the Soviets were deploying T-64s into service with composite armour well before the Challenger I or M1A1 Abrams or the Leopard 2 got anywhere near service...
Don't know about T-90MS armor but older T-90A still uses old armor technology which only gives ~500mm of RHA against HEAT at 0 degrees, as compared to ~1000mm of modern western tanks. Relatively its thickness is higher because the armor is heavily sloped.
Perhaps you need to source more reliable sources for your armour specs.
Note that even some Russians tankers don't like T-90 so much and prefer older T-80U.
I suspect you mean some Ukrainian nationalists like the T-80U over the T-90 because of national pride.
The T-80U has a different underfloor ammo storage arrangement where the propellent stubs are kept vertically and are not covered or protected from the crew compartment.
This means that any penetration of the turret area including sides and rear would result in a shower of sparks and hot burning material showering down on cardboard propellent stubs filled with propellent and designed to be consumed when fired. The result of any crew compartment penetration resulted in all of the propellent stubs igniting at once which as you might imagine is not good news for the crew.
The T-72 and T-90 series on the other hand have an armour plate over top of the propellent stubs that are lying flat and parallel to the floor so sparks and ash and hot metal would not reach them.
Talking about the slope of the armour is interesting, but only a fool would not slope their armour to further improve protection. The layers inside Dorchester armour and Chobham armour before it were designed to break up long rod penetrators like APFSDS rounds.
APFSDS rounds cannot be deflected by sloped armour only full calibre penetrators can be deflected by sloped armour and they are not used today except by heavy artillery (like the 152mm APHE shells still carried by Russian artillery).
The fact that Russian tanks manage to get the same overall protection as western tanks by using ERA and by sloping their armour yet in a tank that is 20 tons lighter than the western tank is very amusing.
Are you also going to criticise them for having a big powerful gun or effective new thermal imaging night vision devices too?
Are you going to complain that their 50 cal HMGs don't have to be head spaced or the timing checked every time they change a barrel like the old M2 US 50 cal?
How about Shtora... are you going to suggest it is inferior to western equivalents because they don't exist and therefore it is clearly a waste of time and money?