As Jurassic mentioned, unsprung weight makes a big difference to how well a rear suspension system reacts on the trail. More expense and more failure? Pinion gearboxes are well proven to be super reliable and very low maintenance, the part that would be the most unreliable would be the motor and all Ebikes have them, followed by the chain, derailleur and cassette and you have just removed that from the equation all together. Even if you still run a chain drive, is works like a single speed, so you can have the perfect chain line, it doesn't have to be flexed sideways to go up and down the cassette, plus you can run something like an 8 or 9 speed chain which is stronger and cheaper and with all this, will last forever.probably adds more expense and more failure to the most expensive part of the bike.
I'm not convinced the backwheel really feels lighter either, that sounds like snake oil and seemed to be the thing he was most incistent on getting across more than anything else for some reason.
A friend of mine has a Pinion gearbox in his Zerode and he loves it. Hopefully they do a Ebike version, and I'll be straight onto that.