I had a buddy into BMX sidecar racing back in the late 70's BMX scene. The hacks were very well made and burly stout to handle the monkey climbing all over it at speed! The riders were all completely insane, with the monkeys far worse than the riders as a rule!
I think it would work just fine once you got a feel for it and found someone athletic (and crazy!) enough to be your monkey.
Stuffing a HT motor in one would be delirious fun!
I say go for it! You'll never pull up to one like it at a light....
Here's some inspiration:
If more is better, then too much is just enough.
If you can't afford it, build it yourself.