Thats a matter of trading strengths and weaknesses.
Your beloved Nashorn has all its strengths in one spot, long ranged combat against tanks, and it comes at the cost of being very weak in other areas, being open topped and slow.
Meanwhile something like an M4 Sherman is more of a middle ground approach. It isn’t the best at anything but its capable of most things. The gun can’t pen everything but it can pen most things if you know where to aim. Its not the fastest but its not slow either. The roof 50 isn’t as good as dedicated SPAA but it can take out planes when used skillfully.
Shifting to planes, the Pe-8 with its massive bomb can wipe out many tanks but, even with one of the better defensive armaments on a bomber, is pretty easy to kill with fighter. Most strikers trade dogfighting capability for more ground pounding. A fighter-bomber falls between a striker and dedicated fighter, the latter of which typically can’t do much against ground vehicles that aren’t open topped or very light.