Guitarists, knowingly or not, hate flat keys because each flat in a key signature removes an open string as a note choice. This literally makes it harder to play since the musician must fret more notes.
Let's take the G chord vs. the A flat chord as an example. Using the six string first position version of G we have three closed notes and three open strings.
In the A flat chord, no matter how you swing it, there are zero open strings.
Here's a typical way that a guitarist would hold an A flat chord:
Here's a crazy way for a guitarist to play an A flat chord:
If you thought the A flat was difficult to play consider that we don't usually play flat chords inside of sharp keys. That means that if you're playing an A flat chord it's likely that you're also playing in a flat key with other flat chords that are equally difficult.