While I'll agree on the weakness of the cheesy rims, aluminum bikes - even wallyworld ones can be fine for motorizing. The key is to use a careful eye when picking out the bike to be motorized while remembering that motorizing it will put far more stress on the frame than it was initially designed for - which is equally as true for steel as any other material.
As an example I suspect any
featherweight road bike would get shredded around here regardless of composition were it subject to a sustained 30mph with our roads, even without an engine lol, whereas my gusseted aluminum motorized mountain bike shows no signs of damage or fatigue after over 5000 miles of outright abuse despite the fact it's a wallyworld bike...
It's just not the cheapest one they sell & it was chosen specifically for it's beefy yet lightweight frame