The brushless multi-phase DC motor requires quite sophisticated electronic control systems and power inverters
The technology is well established. It is widely used in everything from model aeroplanes (whose brushless motors are as powerful as highly tuned nitro-powered engines of similar size/weight) through industrial controls, right up to very big stuff.
It's the batteries....
I disagree re the market for a 2-4 seater with 4hrs endurance - after all that is all a spamcan does, with "PPL training" fuel management. The problem, even if batteries were available, would be the energy requirement. 100HP is about 75kW and for 3 hrs that is 225kWh. This is cheap enough - about £20 - but at 415V (3 phase) is a lot of amps. If you want say a 3hr recharge that is 75kW drawn (assuming 100% efficiency!) and that is just one aeroplane.
Forgetting real long distance cruisers and looking at low power low weight cases (which is where most electric propulsion is aimed at) one could do stuff like solar charging; for example a private owner who flies once a week could largely solar charge (in the summer) with photovoltaic panels costing as much as the plane itself.
There is a long way to go...