Assuming you're aiming for 60hz 1440p, then you will be able to run every game with playable frame rates. More demanding games will require you to drop the detail settings down, sometimes quite a bit, to stay near 60hz. But as long as you're okay with that, then a 390 is up to the job for sure.
Some people won't consider a card suitable unless it can achieve very-high to ultra settings in every game, which would be a big ask for a 390 at 1440p... but that's not my view.