Well, I've seen the IL-2 take direct 37mm hits to the mid fusalage and inner wing sections and keep on trucking. I've also hit them with the tiffie, and they ARE tough. The tail is definitely the weak point. If you hit it anywhere but the outer wings or tail it will absorb more than a 17 can take in 20mm.
I've also noticed a lot of people flying them badly. IE, rolling, coming in low and slow and straight. Making easy pickings for an ostie dweeb like me. And making the 23mm next to ineffective except for detracking armour.
I think the IL-2 issues relate to how people are flying it more than anything else.