A couple friends and I built a mobile app (Chess Puzzle Blitz) where we allow people to compete for fun or money solving chess puzzles.
We have a pretty large puzzle database but are always adding to it.
One thing we keep iterating on is the types of puzzles we surface (currently only mates 1 through 5 (usually 1->3). For example, one feedback we got early on is that black puzzles are harder because of the mental switching between puzzles (so we rate black puzzles harder in that context).
For you, what makes a chess puzzle a good puzzle and what elements make them hard/frustrating?