Not to start the "debates" again, but the aluminum bike is much maligned for little reason - I've one that's not only been beaten relentlessly for thousands of miles but also has... *whispers* a drilled D tube OMG *gasp*
While I don't
recommend drilling the frame (I was forced to by limited clearance), the only thing that's a problem with an aluminum bike is chafe - anywhere there's stuff rubbin' could eventually gouge. Other than that - it's the same as judging any bike to see if it's suitable for motorizing, simply check the welds to make sure they did a good job.
I will say that just about any
bike gotten from WallyWorld is gonna suffer the same problems - junky hardware. Shifters, rims, that sorta thing. Most of the frames themselves are pretty good.