I think most machines have a certain or recommended adjustments for using fmq. So some fiddling is normal.
On Coats and Clarks threads I must disagree, I have and use many different threads for different things and I have found that C and C is a good thread, my machines like it and make great stitches and it is strong. So I think maybe for some people they don't like it for their reasons, but I also know a lot that do like it including me.