I currently own a Retroid Pocket 3+ and it's been my go-to handheld, mostly trying to get some of the harder-yet-possible-to-emulate systems to work. If you're like me, you'll probably need some help (SOME) getting this set up, so that's why I recommend this Retro Game Corps article that also covers the Pocket 2, 2+ and 3.
Now obviously everything from the PS1 and before that will work, except possibly the Sega Saturn and 3DO in a few cases. Dreamcast, PSP and N64 emulation seem to be on-point, especially given how crazy trying to emulate those systems still is. Though so far, aside from Rogue Squadron for N64 and SSX On Tour for PSP, I've had no issues with those systems.
And notice I said 3DO, which aside from some audio issues running Alone in the Dark via Retroarch's Opera core it's been good so far. I might try the DOS version, possibly running via the DOSBOX Pure core, which I could theoretically use to map keyboard and mouse functions to the controls.
PS2, Gamecube and Wii have been plenty of hits but a good deal of strike-outs. More or less, it's what's been said on this spreadsheet. The TL;DR version is being able to run a handful of these games are an added bonus and should NOT be a major selling point. And while some games run well at first, as .hack//INFECTION did, DO expect some issues like random freezings.
It's also worth noting that there's a special version of Dolphin you can install called "Dolphin for Handhelds" when you first set up the device for the first time. This version apparently balances performance and compatibility, and there's the Play Store version and two additional forks.
Sadly, the PS2 emulation comes via a recently abandoned emulator called AetherSX2. What I usually do is if it lags a bit, I'll open the settings and set them to fast. This just turns on certain hacks and fools around with some settings to get it running as close to full-speed as possible.
Outside of emulation it's a pretty awesome Android device. With the Play Store, intuitive onboard controls and an excellent built-in keymapper, more often than not you'll get great results with games. And it's also got Bluetooth support and HDMI out (there's a mini HDMI port on the device), so you can use it on the TV, and given it's 4.7" 16:9 screen that might come in handy. Though keep in mind the keymapper won't make EVERY game work like magic. For example, due to how Call of Duty: Black Ops Zombies functions, mapping controls was too much of a hassle. And while the Call of Duty Mobile app works, you'll definitely need the keymapper since the onboard controls aren't recognized.
And as for the launcher, you can choose either bog standard Android 11 OR the unit's proprietary emulation frontend, though if you're like me and so many others, you'll probably want to go with Daijisho or any of the other frontends out there.
But so far, it'd handled everything pretty decently.