You would be surprised what shit people sell. I could be here for a damned long time saying what I have bought which in all honesty was a waste of money, not to mention the extra time (I have little spare!) and hassle but I shall keep it short and sweet . As long as there are people to buy it (there always will be purely out of principle) they will continue to be sold (I know of people who buy lower arms yearly on cars because they are too tight to buyer genuine items or poly their existing items). In a nutshell here are things that have gone wrong with bits I have bought:

Q-Drive Drop Links: On my 205 all 4 boots had split within 2,000 miles/2 years; an MOT failure
Engine Mount: Same story on the above car (the sleeve fell out after 10k when I came to replace it (the Group N items and genuine items were never that bad IME).
QH Thermostat: My Triumph Spitfire never ever got warm, handy in winter
Alternators: Don't even get me started ; one had my MkIV XR3i stranded in Italy amongst the countless others I have changed on others.
Coolant hoses: My Stag (now almost sold) has had almost all of the 'new' (now 3 year old) coolant hoses changed for silicon items, either due to them fitting crap (and they leaked as a result despite many attempts at fiddling with them) or them cracking and hardening and hence not sealing well at all. I dread to think how much I have spent on coolant (OK, not much) and wasted time in checking the coolant level; The buyer of my car noted how in the Rover circles shit quality bits are rife.

Like most things you get what you pay for IMHO . I guess when I was doing 40k a year I got sick of working on cars besides general servicing.

[/rant over]