Unlike Remix OS, Phoenix OS is recently released and comes with Android 7.0. Remix OS support is discontinued and no further updates can be expected. But Phoenix OS updates can be expected. UI is same for both OS.
There's never as good a price as a grand total zero dollars, and that is exactly what this will cost you. That means that if it does not work, if your not happy with it, or if you just cant afford buying an operating system, you can still get RemixOS.