Chris & Derek,
I have quite a bit of experience of running three types of twin, but not those you mention...

First of all, all the original Taplin Twins. Derek commented kindly on my video. The engine simply runs perfectly, and so do all my other TTs.

Secondly, I've run a Micron flat twin quite a lot. Again, it runs perfectly. The comment in Mike Clanford's book is totally unjustified concerning the Micron flat twin. It's a super engine.

Lastly, the best of all was the Retro Twin, made by André Demesse. These were the best twins I've ever seen and, strangely, you'll see that the (PAW) carburettor is "at one end", and not between the cylinders. Operating the throttle on this engine was like blipping the throttle on a Ferrari. There was no audible acceleration phase, just "instant rpm"!
Unfortunately, I stupidly traded this engine, certain that I could get André to make me another one and, sadly, he died...

Oh I forgot, I also had a Delmo twin, but could never get it to run properly... and an inline Ross Twin that I never tried to run.

However, the Taplin, Micron and Retro were/are "no problem" super runners, so all model engine twins certainly don't have problems.

Here's the Retro (5cc) - Excuse the photo quality, they're scans of 20-year old prints: