I tried toner transfer and was disappointed with the results.
Using my laser printer (Brother DCP-1612W) I could never get a dense enough print on to the transfer paper.
It looked OK to the eye, but when heat transferred to the copper clad board it was obvious that there were "tin bits" in the toner film on the copper.
As a result, when etching, the wanted copper areas had a mottled appearance and thin tracks could have no continuity.
I abandoned my efforts with toner transfer and have adopted milling machine techniques, where I have had some good results. I still need to develop my skill with the milling, but I am getting there.
From reading on the interwebs and a discussion with someone I met who knew a bit about it, it seems that a good printer to use for the toner transfer technique is the old HP LaserJet series.
JimB