A channel wing is essentially a very blown wing. The props pull air through the channel creating a low pressure area. The channel is both a guide and prevents air coming up from the high pressure side.

It's remarkably effective. Custer's first prototype was damaged when he was taxiing it around an airfield and accidentally took off. Custer was not a pilot so bent the landing gear when he came down hard. On paper, if the engines are powerful enough and you have an appropriate control system, these are VTOL.

There is a 'but' though. They are very, very good at going slow, and very, very bad at going fast. Drag and weird interactions between the channels and the props cause ~~BN Islander~~ dangerous levels of vibration above certain speeds.

Plus, if you lose power whilst slow (i.e. when landing or taking off) the aircraft takes on the aspect of a brick, drops out of the sky, and kills you. Losing one engine means it flips over and super-kills you.

The biggest problem (IMO) with them is that they don't really have a use case. If you need that sort of short take-off performance, you get a helicopter. If you need a reasonably fast 5 seat twin, you get a Cessna 310 (or whatever).