There are about 20 such concepts out there, the big players like Boeing and Airbus are playing only because they don't want to explain to their BODs how they got blind sided by some tech startup.
First off, they are not a threat to airline aviation (maybe a threat to taxi drivers, if uber has left any of them alive). UAM vehicles MUST have VTOL capability and be battery or hybrid powered. Not compatible at all with long-range, high-speed air travel.
The reality for all of them: There are numerous hurdles and problems....
Safety. They need to be VTOL, so most concepts cannot glide to a safe landing. Will need alternative safety features (CAPS ?). Since they will be endangering the public on every flight, they will need safety statistics which are FAR better than GA/135 stats. GA/135 mostly just endangers the pax. But UAM's will be flying entirely over densley populated areas. They will need safety on par with airlines, which does not exist at all in small, light, GA aircraft. That sort of safety is achieved in large part with redundancy, which adds weight and cost.
Certification. See safety. 91/135 standards will not be enough, for either the FAA or underwriters.
Pilots. Initially they will need pilots. Even if they had automation technology (they don't) they would still need pilots as backups until they can convince the government (the one that's shut down) to go out on a limb and certify. Pilots are getting expensive and hard to find.
Got all that figured out? Now it gets even harder...
Production. UAM requires tens of thousands of aircraft, so you need to build them at a rate like the automobile industry. But to certification standards like part 25, which is quite different from automotive or other tech. Nobody knows how to do that yet. They can probably figure it out but it's going to be harder than the visionaries think.
Public acceptance. I'm not talking about people using them, they will. I'm talking about for every wealthy commuter who can blow $70 each way on his daily commute there are at least 100 not-so-wealthy folks who don't want flying cars buzzing around their neighborhoods. The only way you're going to solve that is by making it so cheap that everybody can use it. Maybe possible with economy of scale, but there's going to be a very big profitability valley to bridge. When it starts out as an exclusive perk for the wealthy, the NIMBY masses will revolt in a big way.
I'm sure there are problems I haven't thought of. Also confidant that somebody can solve them all, eventually. But it's going to be an uphill battle on the economic front, as well public acceptance... that last is not very predictable.