The correct procedure for making a complaint is to send it to the Customer Relations Team. Unless you work in that department you don't see a lot of complaints.
There are also a number of complaints posted publicly on Twitter but considering the size of the Northern operations there aren't that many complaints posted - especially compared to Metrolink.
Oh and if you want to tell someone to find something better to do with their time (as you told me the other day) why not say it to the person who runs this Twitter feed:
https://twitter.com/metrolinkFAILS who must a significant amount of time each day re-tweeting complaints about Metrolink.
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Northern have a similar number of 156s and 158s but the 156s are split across 2 depots whereas the 158s are all based at Neville Hill. Also the 158s seem to predominately stay on Yorkshire routes while a Heaton based 142 can finish up on a Penistone Line service or a Newton Heath based 150 can finish up on a Sheffield-Lincoln service.