Hi Skip,
I use Uni 6/0 and 8/0 for a good deal of my fly tying and have no complaints with either, in fact I find both threads to be highly acceptable. I don't use 3/0 often and have only a couple of spools of Uni in this size, but my 3/0 chartreuse thread has been a constant source of problem with my whip finishes. I've approached this difficulty from a number of different angles and it seems to me the weave or twist of the thread leaves a highly rough, abrasive surface which tends to bind during the final stages of the whip finishing process and requires more and more pressure to finish the knot. The thread may either break from this closing force or just cut itself as you pull the knot tight. My solution is simple----half hitches. A little bees wax might also help the knot close. My favorite 3/0 thread is waxed Monocord which glides closed on whip finishes. I'm trying to use this 3/0 Uni Thread up and will not purchase more. Just my 2% of a dollar. I'm sure others will disagree----they always do! 8T