I got hit in Level 12 and Level 27. The difficulty is not linear. The 30th wave was much harder than the 36th wave for example.
The difficulty rose incrementally in the beginning like a game generally should, but it dropped near the end, unless those were intended to be "cool down" waves.
The shotgun seemed to delay in firing and seemed to "fall behind" when you fired numerous rounds.
Overally it's style is good, but it's gameplay is average.