It's impossible to tell w/o lookin' at the bike... but I doubt the vibration from a properly mounted engine running would be the cause with most fastener failure. Here's what I've seen to be the most
Attempts to soft rubber mount.
Gradual over-tightening of fasteners by "checking torque" w/o a torque wrench.
Failing to use lockwashers.
Frame damage/mismatched mount radius.
Neglect and/or improper installation.
Rear sprocket not being centered.
Damaged rear wheel, out of true/sloppy bearings
Slack chain/tensioner/misaligned sprockets.
I'm sure there's other causes as well - but the above are defo the most common. There's some truth to the claim that the hardware itself is substandard - but given my aluminum Schwinn has thousands
of miles on it with the stock "rag joint" sprocket, all stock fasteners, and even has a drilled D tube (that IS a sketchy lol) and has survived a buddy's EPIC crash as well as the relentless abuse by yours truly... Given there are far
more people that haven't had engine mount issues than people who have...
I've no choice but to suspect other causes