The crankset arms are probably plastic and will quickly degrade and become loose, the bike will then be useless and require a replacement crankset or arm, which is time and money and hassle.
They're going to be plastic coated steel. Heavy, but people use cheap bikes to go to work all the time.
If you buy the bike and don't assemble it properly it's possible that the crank will deform because it was loose on the bottom bracket spindle and then become useless, but that's not quite the same thing.
A cheap bike can be fine. I see lots of people riding them to train stations. When I used to live on a narroboat, I knew a man who bought cheap BSO type bikes and instead of locking them up would tie a rope round them and chuck them into the canal. He was still better off replacing the bikes every couple of months than he would have been buying bus tickets.