I don't have the expansion but I can say my personal experience with Flick 'em Up is that I enjoy it but have difficulty getting it to the table. I bought it on the assumption that it would work well as a big group game, but ended up really disliking it with more than 4 players. It's also been pretty hit and miss with our group (pun not intended). The included scenarios are pretty bad, so it needs some fussing with to get the fun out of it. The rules for movement / shots / etc. are also a little wonky and unintuitive, which has lead to some people just not liking the game at all (thinking of being able to block doorways with dead cowboys, hitting something counting as missed movement, and the infamous "no thumb" rule here).
There was a thread recently on r/boardgames where a bunch of people shared similar experiences. I wonder if a sufficient number of people just found the game to not fill the niche they thought it would.
Yeah. All of this. Oh well