Seems like a very good option and if there is a fault with the unit just remove it and carry on using the bike. At around £1000 its a premium product as many whole ebikes retail at similar or less prices. You can get ebike kits for bikes including batteries from about £350 like Yose power kits. Also its clearly limited to disc only bikes that can be converted and probably fairly standard frames not 20" folding bikes with disc brakes etc.
I understand its far less stressful to the stays compared to disc brakes but it could add another frequency of fatigue to the stays to shorten their lifespan. Fatigue is all about flexing and I can't see how this wouldn't increase flexing of the left rear stay, yes the flexing would be less than hard use of your rear disc brake but it would be enabling and cutting off power quite often. Probably not an issue unless you have a very lightweight performance road bike.