I don't find my 820 finicky or tempermental at all, certainly not about thread, but then I'm not that adventurous yet.
I HAVE used every weight of cotton thread and several brands. My current favorite piecing thread is Presencia 60wt, followed closely by Aurifil 50wt and Masterpiece by Superior-the "old" 2 ply, haven't tried the new 3ply Masterpiece. Mettler is ok, too. I've quilted with 12, 28, 40, and 50 wt Aurifil plus Perfect Quilter and King Tut by Superior, which are both thicker.
I don't think I've tried silk thread in the 820 yet, and the only poly I've tried in it was Mettler Metrosene. I'm going to try So Fine in the future.
There are tips and tricks for taming naughty threads like metallics. Don't worry!
Btw, I use needles from 60/8 to 100/16 with no trouble, BUT you can't use the autothreader on needles smaller than 75/11. One reason I like the Presencia is that I can use it with a 60/8 needle.