simple

It depends, a lot of modern AAA games run fine but a good amount of them are very unoptimized and run poorly on the Deck.

Valve said not to expect a hardware refresh soon since newer chips are more expensive and not that much faster, just look at the handheld PCs selling for ~$900 that are only 25% faster.

There are plenty of benchmarks online so you can check if the games you want will work properly or not.

With the recent news that Nvidia are preparing for ARM gaming laptops, things are looking very bright for arm.