The orangepi build of retropie is very sketchy right now. I assume these retroengine people are polishing it up, but even if they manage to get it working as well as retropie on the RPi3, what's the point? Retropie on the RPi3 already exists
They're not selling you anything that you can't buy today, except for that "custom" case. If you don't care whether or not your ARM-based emulation box kinda looks like a silver Sega Genesis, there's no need to wait for this crowdfunded doodad that may or may not actually be delivered.
- RPi3 w/ case, heatsink, and power supply: $50
- 16GB microSD card: $6
- Cheap USB dualshock knockoff: $8
- Fully functional, polished, and regularly updated Retropie: Free
For a little less than the cheapest still-available retroengine tier, you can have a better-specced board with better support, software that already works, and you can have it in two days instead of six months. It's kind of a no-brainer.