は・
This construction concedes that A would at least be understandable or tolerable, then presents B as clearly worse or harder to accept. A is the less bad case, not necessarily a good one.
The comparison always moves from bad to worse
The speaker first allows A as a limited exception: it may be inconvenient, imperfect, or undesirable, but it remains within an understandable limit. B crosses that limit. The contrast may involve amounts, frequency, consequences, behavior, or different categories of people and things.
Clear scales of severity
Notice that each first case is only comparatively acceptable.
二、三日の遅れはまだしも、一か月となると計画を見直さなければならない。
A delay of two or three days might be manageable, but if it becomes a month, we will have to reconsider the plan.
Analyze初回のミスならまだしも、同じミスを三度も繰り返すのは看過できない。
A first mistake might be understandable, but repeating the same mistake three times cannot be overlooked.
Analyze予定が一日延びるだけならまだしも、納期そのものを守れないとなれば話は別だ。
A one-day extension to the schedule might be tolerable, but failing to meet the deadline itself is another matter.
AnalyzeChoosing between the two forms
はまだしも works well when a noun is being contrasted directly with another case. ならまだしも is more flexible: it can follow either a noun or a whole plain clause and means roughly “if it were A, that might be understandable, but...”
Similar ways to make an exception
まだしも explicitly ranks A as the less bad case. いざしらず more detachedly pushes A aside as an exception and is more typical of formal writing. ならともかく broadly means that A would be another matter, without always emphasizing a bad-to-worse scale. はともかく simply sets A aside for the moment and does not necessarily judge it to be tolerable.
More examples
千円はまだしも、十万円はとても払えない。
A thousand yen might be manageable, but there is no way I can pay a hundred thousand.
Analyze少し高いくらいならまだしも、予算の倍では手が出ない。
A little over budget might be manageable, but at twice the budget it is out of reach.
Analyze古い機種ならまだしも、新製品でこの不具合は納得できない。
This defect might be understandable in an old model, but it is unacceptable in a new product.
Analyze知らなかったならまだしも、説明を受けた後で同じことをするのは許されない。
Not knowing might have been understandable, but doing the same thing after receiving an explanation is unacceptable.
Analyze味はまだしも、衛生面に問題がある店には行けない。
Poor taste might be tolerable, but I cannot go to a restaurant with hygiene problems.
Analyze擦り傷ならまだしも、骨折の疑いがあるならすぐ病院へ行くべきだ。
A scrape might be manageable, but if a fracture is suspected, you should go to the hospital immediately.
Analyze雨だけならまだしも、強風まで加わればイベントは中止せざるを得ない。
Rain alone might be manageable, but if strong winds are added, we will have no choice but to cancel the event.
Analyze