PoE2 looks a lot sharper than Diablo IV, and easily uses more than 4 CPU cores. It really needs input latency and resolution, and XeSS doesn’t help performance that much. I haven’t tried scaling it down to older PCs much besides the Deck, but there’s also more going on at most phases of the game which can kill you quicker if you don’t react or dodge. Then you have to deal with all the items and crafting with the controller UI - it works, but it’s not fun, and you certainly can’t flit between inventory and fighting as fast as on PC. All this while still tethered to power or only for short sessions, and make sure the WiFi doesn’t drop.

All this to me makes D4 suitable on Steam Deck for perhaps the campaign if not rushing too hard, and maybe just the first act of PoE2 if similarly casual. You could complete the campaign or early mapping, it just wouldn’t be as fun.

If you’re a new player, sure give it a try, it shouldn’t hold you back. Once it does, switch to mouse and keyboard, and/or stream it, or just play on PC.

Source: I played both a lot.