But if we're talking any kind of mobile/mechanized war, they're useless defensively. And offensively, they're just useless period unless employed in true human wave/overwhelming numbers, and this isn't nearly enough for that.
So that is the problem the Russians face. The total length of the front - especially when you consider Kherson and Zaporizhzhia oblasts - is far too long to have the kind of dense, continuous front those troops may be able to defend successfully. There were literally millions if troops covering the same length frontages in WW2.
So maybe sticking them in trenchlines in Donbass can work, or defending the Crimean peninsula proper. But otherwise, they'll be speed bumps, and that's about it.
Of course they can.
500+ militia - MILITIA - held off 6-9+ thousand of Ukrainian forces for nearly two weeks causing huge causalities and kills upon the Ukrainians before withdrawing.