yesn't in case of Russia a lot of their military just "didn't work properly"/"was ineffective in a very fundamental way"

this wasn't the case for the US, their military did work and was effective in context of the specific think they where used for. It's just that the thing they where used for isn't effective in winning the kind of conflict the US did start.

while this is "just" the "non effective" part being some where else it is a very fundamental difference in the end

like to put it this way,

if weapons of mass destruction like nukes, bio weapons etc. would magically disappear an invasion and takeover of (part of mainland) Russia would be relatively viable (mainly by China, but others might want a slice, too).

on the other hand even without weapons of mass destruction trying to invade the mainland US still seems futile/to be a losing battle, no matter who tries it.