Thank you for the kind comments and feedback!
No, you're not missing some hidden mechanic. I actually have similar thoughts with you about the bomb types. The 4 way bombs basically make the other bombs useless in the shop. Unfortunately that thought came to me a little bit too late, and I don't really have any other workaround back then to solve that problem, so I just leave it as it is.
I'll think of something to fix that problem. But, as of right now, the weaker bombs are there to serve as "fluff", if you can call it that. Basically to make it so the more powerful bomb types show up rarely in the shop. Which, in hindsight, is a little bit annoying and kinda bad game design xD. I'll try to think of something better.
And yeah, the difficulty is not balanced right now. It will get fixed (hopefully) in the future, when I continue developing the game.