It’s not the .40 they don’t like, but the 10mm. The 10mm was developed for them, the idea being that they would adopt it, then police forces would as well, and then it would become ubiquitous as a civilian self-defense round as well.
It performed fine in the FBI’s tests and met its criteria, but they thought it would have too much kick for smaller/female agents so they ultimately rejected it as not a good everyday carry round.
The thing is, it offers .357 Magnum performance in a much more controllable package. Carrying it around is obviously irrelevant, and since pistols are already inadequate manstoppers, when you need the firepower the last thing you want is a truncated version. I figure what they meant was it’s a more fatiguing round to fire multiple dozens of times in practice at the range, to which I say “so what?”
I’m a big fan of the .40 as a great way to combine the strengths of 9mm and .45, but I’ll definitely own a 10mm some day because there’s really no topping it.
I picked up a Glock 10mm as a curiousity - it is a good round with a lot of punch. You’ll like it.