It would make sense, but that seems rather troublesome to implement...
Never let that be an excuse.
Mostly true. Especially relevant in this case.
Absolute difficulty is a non factor up until the point the programming language or media medium starts to groan.
Relative difficulty is only a factor to decide between alternatives (useless illusions, temporary effects, permanent effect).
I prefer the illusions that are partly mental. This results in the target (the opponent) causing the damage that the illusion supposedly dealt.