I have an Anbernic RG40XXH and it's a solid option. But there is an important caveat: the hardware from the device has been upgraded since the first models hit shelves, and in more recent ones there are important software issues. In my case, to turn it on I have to plug it in to a power socket and the battery drains fast when turned off (four/five days for the battery to be completely drained from 100%).
This happens with both stock OS and Knulli Gladiator II. Still haven't tried MustardOS.
I have hope with the new Batocera released a few weeks ago, the Knulli developers will revisit their OS.
Apart from that, it's a very solid device.