Oh ya, he's not very popular in PvP. Partly due to his rarity but also because, even at maxed skills it's hard to find a good team for him.
I know one team that was very difficult to beat and that was raccar's. He's got the maxed skills rainmaker too alongside two AGS's and a dog of war. My defense is inspired from his with the other AGS replaced with an AoS. Since I'm low in the ranks for most of the week I get a fairly alright "defense hold" rate when compared with my previous standard taunt team.
Despite that rainmaker still is a valuable character to have when it comes to your offense team. Suddenly gunner CR doesn't seem so OP any more!!
I won't blame you if you choose not to join the max skill rainmaker gang (shoutout to raccar and ill!)

skill shards don't come like rain but like tiny droplets! Hey, there's a spot available on my list if you're looking for troopers!