I mean I saw my reflections when I stepped over the mirror shards so I figured that by avoiding them, there wouldn't be any reflections of mine to place bombs, but to my disappointment, the bombs appeared regardless of where I was, and also, the bombs appeared even where there weren't any shards.
Yes, it would have been quite a mess with all of the player trails being visible, especially in a bullet hell, so it was a wise decision to make them only barely visible in the shards. I think the reflections look really good, though; layer masks are quite an advanced thing so I'm impressed you got that to work.