One weird trick…statisticians hate this guy!
A minority of games reach 13th level. Of the parties that reach 13th level, a non-negligible amount of them do not has a caster that can cast Simulcrum. Even among parties of the correct level with access to the spell, there are many reasons a player may not cast it (not interested in the spell, did not have the components, does not have 12 hours to cast, etc.).
So, it is fair to say, that except for a minority of the minority, Simulcrum does not figure in their evaluation of the design quality of the game except in the abstract.
However, among parties that have reached 13th level and that have casters with both access to Simulcrum and an opportunity to use it, it seems that there is a high proportion of tables that ban it, nerf it, or otherwise consider that it unbalances the game to an unacceptable degree. So, among those for whom it is a relevant concern, it seems that Simulcrum is not a well-designed spell.
Just another reason that the popularity of a game is irrelevant to determining whether individual components are well-designed.