I pulled this Bushtec around everywhere with my 89 Ultra Classic (Evo motor). Never had any problems with the motor. However, I did twist the transmission main shaft in to two pieces. The MoCo covered it even though I was the third owner and long time out of warranty.....they replaced it under the good faith customer service policy.
It's nearly unheard of for a transmission main shaft to twist in two....even on drag bikes. I've never heard of one before or since. But, in fairness, I was much younger then and I used to do little wheelies with this bike and I'd do burn outs with the trailer on it. Many, many people have towed trailers and sidecars for years with these bikes. Most who own a trailer (unlike a sidecar) just don't have it hooked up that much in comparison to total miles ridden.