Interesting…….quite a lot of members at Epping Forest Horological Club have recently aquired cheap(er) chinese ER sets and, while results are indeed mixed, I have noticed some repeating problems which can be listed under 'user error'…. Firstly, people often claim poor manufacture but when one of the more experienced members examine their set some of these defects are reduced or simple go away.
Firstly, the large number of slots in ER design allows debris to quickly upset accuracy as just one tiny piece of swarf will prevent the slots closing evenly and start to spoil accuracy. Next, people commonly try to mount the collet imperfectly and don't realise that there is a little ceremony in fitting the collet into the tensioning nut correctly first. Finally it's worth mentioning that in industry ER is used to hold cutters and its very common to 'clock 'n tap' to get proper needle placement by using a small hammer or mallet.
Additionally, it's worth mentioning that one has three options for ER collets, namely an ER chuck fitted to spindle,….a Morse taper ER mandrel that fits into the lathe throat and finally an ER fitting in a square or 6 sided block. For my small lathe I chose ER25 in a square block which I use in my 4 jaw chuck…..the drill is to grip the block, fit collet with silver steel piece, clock, adjust for accuracy…….much better than the 4 jaw on it's own IF you are going to do a session of collet work where you will need to make several changes of size etc., also, the ER is much better with the small sizes I use in clock making.