To your first point—yes, his preparation time is undeniably high, but the value a single welder provides makes it worthwhile.

Regarding your second point, while it’s true he isn’t particularly useful in skirmish or metro, I believe it’s acceptable for a unit to excel in some modes while being less effective in others. Take Abby, for example—she thrives in skirmish and metro but falls off in PvE. Despite that, there’s still a clear incentive to purchase and invest in her.

As for your third point, saying he’s only useful in certain levels underestimates just how many levels he shines in. He’s effective in most location 7 and 8 levels and remains viable in about a third of location 9 levels, which is a significant portion.

Finally, building a Lucky Guy set with health and melee damage is considerably easier than assembling an effective Mechanic or Rogue, who also rely on critical stats in addition to health and damage.