IMHO the biggest problem is that it is simply awfully hard to enter the GA market.
Cirrus did it some years ago, by marketing at high net worth low experience individuals (the only avenue left if you want to shift capable tourers).
Diamond did it, with the mainly the avtur burning engine being the chief selling point.
There is a lot of activity on the VFR-only "sports" / "lawn mower with a wing" scene.
Not a lot else going on. Some other projects have been shelved (Evector Cobra for example).
I think this kind of project is going to require massive funding, to end up with a reliable car and a reliable plane. I would not want to be driving a Reliant Robin around, with Vauxhall Viva electrics and 3mm of flimsy fibreglass protecting me from any idiot that runs into me, which is what the "car" portion of this will inevitably be.
It doesn't solve the #1 GA issue which is lack of airports, airport closing times, lack of IAPs, etc. It merely solves the issue of transport between the airport and the place you are going to, and for the money involved one could get taxi rides, or a rental car dropped off at the airport, for the rest of one's life.
There may be specialised applications but the volume will never be high enough to make it viable and to create an incentive to iron out the inevitable bugs.