I used to be a fairly hardcore mountain biker until I got too old and developed a lot of medical issues, among them fairly severe arthritis and fibromyalgia. I broke 2 aluminum frames, one Trek and one Gary Fisher. Both broke where the seat stays attach to the seat post. I do admit these were not top of the line bikes, and were abused.
Modern aluminum bike frames hold up very well, as bikes. Motorizing them is a whole nother thing. First of all, I would never recommend a Chinese engine to anyone. If you want a motorcycle, get a motorcycle. These engines are junk, and even on a super strong steel frame, you will have to just keep replacing engines. To my knowledge, nobody makes a quality frame mount engine. Also, the chain tensioners and rag joint rear sprocket mounting that comes with Chinese engines are worse than junk. They will fail, and possibly seriously injure you in the process. Most modern aluminum frames use oversize tubing for strength, and the Chinese engines don't fit them very well. And has been said, vibration can be a serious problem. Aluminum welds are particularly susceptible to vibration damage. IMO all good reasons for not using a Chinese engine, especially on an aluminum frame.
Aluminum frames also present a problem with rack mount systems like the BMP kit. I just bout one of these kits, and it appears very well made, and more than up to the task. And you certainly can't beat the price. But they do have an issue with aluminum frames, as I found out with a similar setup years ago. They clamp to the seat stays, and when I tightened the bolts, it crushed the aluminum seat stays and destroyed the frame. Again, this was not a high quality frame, not even a name brand frame, probably Chinese. But even on a good quality aluminum frame, if installing the mount didn't damage the frame, the vibration from the motor would probably crack the welds over time. The clamp on mount forms a structural part of the motor mount/drive system. I can see using a GEBE setup on an aluminum frame, it uses a thin metal strap connected to the frame, which does not really put any stress on the frame welds, or crimp the frame tubes. But it is expensive, and I don't like the way the pulley attaches to only one side of the wheel. I'm sure it would work fine with a super strong rear wheel.
But I am a friction drive fan, it's all I have ever used, and it works great. I am mounting a Robin/Suburu engine on my bike with a BMP kit. A bit about the bike. This is an early '90s Trek "Antelope 800" steel framed mountain bike I bought at a Goodwill store for $15 in great shape. And when I say steel, I mean STEEL. This frame is super high carbon steel, you would almost have to run over it with a cement truck to break it, it;s that tough. It also came with a perfect set of Araya double walled aluminum rims with stainless steel spokes. Apparently Trek made better bikes back then than they do now. I stripped it down, removed the 2 largest front chainrings, the front derailleur, the rear derailleur, and cut the chain to the right length for the center rear sprocket, making it a very low geared single speed. In a concession to comfort, I also installed a suspension seatpost, and a big wide plush cruiser seat, and higher bars with foam grips, so I could sit upright to ride it. It has been very comfortable and reliable during the past 3 years with an Island Hoppers Viper clamp on friction drive kit, but the cheap 2 stroke Tecumseh TCII engine bit the dust, and the mount kit wouldn't fit a modern engine with the larger clutch. And when I said 3 years, I don't mean daily use, in fact, it rarely got ridden because the engine made more noise than the siren on a fire truck. I hope the modern 4 stroke solves that problem.
I want a motorized bicycle, not a motorcycle look-a-like. I already have 4 motorcycles and 2 scooters, one of them a vintage Vespa that I restored. In AZ, you can legally ride a MB in the bike lane, as long as you don't get assaulted for doing so. One of the reasons I want a quiet engine.
IMO, you should carefully consider your needs and wants, if you need transportation, get a decent steel framed bike, and put a BMP mounting kit and 4 stroke engine on it. If looks are more important, get a decent steel framed bike and put a Chinese motor on it. Buy 2-3 at the time, they don't last that long. If you want a super expensive setup, get an expensive aluminum framed bike shop bike, and put a GEBE kit on it. And make sure you get their rear wheel to go with it. Jerry.
