MC-21, the base of the proposed UAE/Russian JV, is the Russian C919, both new comers to the game.
Besides money, UAE has nothing to offer. If UAE manages to get other (Arab Gulf) countries to support an Arabic technology buildup, then this plane can have a big market (population + area) to cover.
The certifications from FAA and EASA are prerequisite for any plane to fly over or land and take off from USA and EU. They are not a must for other countries. However many non-US/EU countries only accept planes to operate over their airspace that get FAA and EASA certifications because most countries simply have no expertise to do that job. It is the same as the world ship classification agencies.
What CAAC got from FAA is cross-certification. That means CAAC certify C919 on behalf of FAA. That means when CAAC says C919 is qualified, it is qualified in any countries that accept FAA certification.
Now, back to MC-21, Russia does not have such deal as CAAC-FAA. In current and foreseeable future the chance of Federal Air Transport Agency (Russia) to cut a deal with FAA or EASA is near zero for well-known reasons. In this situation, the air corridors that MC-21 can fly is quite limited, this will greatly reduce its market.
Essentially, these certification agencies are like currencies, countries may switch from Dollar to Euro to RMB, but who is going to switch to Ruble?
MC-21 may be a good plane in technical terms, but it just don't have the political, economical and marketing environment to compete with C919 which itself is already facing strong market leaders like Boeing and AB.
MC-21 is similar to Su-57, grabbing some guy with a deeper pocket to survive. We will have to wait and see.