I didn't find the tail fillet all that tough, as I recall. For me it was making the flat front fuselage sides become curved (while cracking one in the process) and then shaping the balsa tree trunks into the bottom of the front part of the fuselage.

After a LONG layoff, I'm finally back to work on mine. Top wing is about done. I did a really crappy job sheeting the center section of it, so I would probably say that was also a tough part for me.

-Scott