That reminds me, the Invincible TTRPG is having a Kickstarter; eventually...
https://www.kickstarter.com/projects/1192053011/invincible-the-roleplaying-game
Oh, we're looking for common pitfalls, eh?? I find expecting your players to read the rules to be a common mistake among designers. How do you design a TTRPG that doesn't require players to read???
WOTC made minions because they thought tracking the HP of twenty plus creatures was too much of a pain.
Does anyone here want to share tips for managing a fight with thirty of so giants??
Crawl around these forums long enough and you'll find people who tolerate the rules of D&D to access the community and reach it has so they can do the immersive fiction creation/simulation that they actually want. I'm not one of them (and pointing out who I think they are could be too personal...
The obviously solution is to have the PLAYER handle a crossbow and dagger IRL and base your in game ruling on how well they use both weapons at the same time.
If you're not willing to TPK your party to maintain impartialness, then you really weren't impartial in the first place.
That's not a bad thing; but you gotta be honest with yourself and with your players.