The $100 machines are junk but I suspect that if many of us got one of them - did some appropriate bobbin and upper tension adjustments and sewed appropriate fabric on it - we'd have little trouble.
The problem with a lot of them is that they're poorly set up from the factory and usually purchased by people who know little to nothing about sewing then fall into 2 categories: dump it without reading the manual but after deciding they don't have the aptitude for sewing, and after breaking a bunch of needles because of poor sewing technique (in which case the timing may be forever ruined). Or 2, see number 1, but they futzed with the machine first a bunch and they will never manage to get it to sew right because the futzing was frustration based, not logic based.
Now, if you take a machine that's in a similar to the price range that say a 401 was in when new (so let's say somewhere in the $5000 range in today's $$$), even though they're in plastic clothes and certainly not as solid as the 401,.. they're very serviceable machines for what they're designed for. Most can go through a couple of layers of modern dress denim. You're still hooched if you want to go through a pair of work jeans though).
I'm recently certified by one of the biggish names and have torn down $10K embroidery machines. I don't love the plastic inside, but most of the plastic is servos and pulse motors anyway. The majority of the important parts are metal in that price range.
I do agree with Joe that this is a worthy pastime. The more we hone our observation skills, the better the machines we get or sometimes the better the value when no one else bids on them.
Things I look for now that I never did before I got a machine with one of these issues:
Broken hook tips
Missing stop motion set screws
Missing Bobbin case spring screws