Considering how wonky the mouse is in this game, I'd rather argument with the F keys allowing your to switch targets faster, or setting multiple skills macroed to directly cast on party member x (the latter being also possible on gamepad). I can't count how many times I clicked with the mouse on a target only for the game to count it as a sliding click and not register it.
You have to hold your mouse perfectly still for it to register your clicks, which bring us to the next point: traveling time. A gamepad' down/down/down will bring you to party member 3 in a set time, which is under 2.5 gcd. Depending on the position of the mouse, the travel time can be longer or shorter, but still under the 2.5gcd. However, the position of the mouse is not guaranteed, you have to hold the mouse still over the party member, which is harder to do in stressful situations for me.
Maybe <mo> macros help with that, but then you will have to have 2 same skills on the bar. Depending on your total skill count, you will have to perform finger acrobatics to e.g. reach ctrl+0. Let go off the mouse? movement loss. Small finger on lctrl, thumb on the 0? I can't do that blind. Remapping helps partially, but I'd still have to look away from the screen to reliably press the correct key combination. There is also clicking skill with the mouse that introduces more travelling time and the same wonky clicks.
So yes, changing targets is potentially slower on the gamepad, but if you organize your party member list intelligently, all the advantage a keyboard will potentially give you is the ability to spam the healing key waiting for the GCD to reload. Prejudices against gamepad players are completely unjustified. People can suck with either, and if you force me to use the keyboard, it will end up very, very ugly.
Only applies to enemy targets. Can't target party members or npcs even if your filter is set to friendly/all.