ならまだしも is used to express that something would be acceptable, understandable, or tolerable under certain conditions (A), but the actual situation (B) is not acceptable, reasonable, or tolerable. It sets up a hypothetical or conditional scenario that would be forgivable, then contrasts it sharply with reality.
The structure conveys the nuance of "if it were A, that would be one thing, but B is a different story" or "A would still be understandable, but B is too much."
The first clause presents a situation that the speaker considers more reasonable or acceptable, and the second clause presents what is actually happening – something the speaker finds problematic, excessive, or unreasonable.
Various word types can precede ならまだしも:
The speaker is not necessarily saying A is good – just that A would be more forgivable or more understandable compared to B. The emphasis is always on criticizing, complaining about, or questioning the appropriateness of B.
This grammar is similar in meaning to ならともかく and はまだしも. All three express the idea that one situation is more acceptable than another, but ならまだしも tends to carry a stronger tone of criticism or frustration toward the actual situation.