UV Clear Fly Finish is an amazing product when used correctly.  Below are some FAQ's.
Sometimes my UV Clear Fly Finish turns yellow
Regardless of brand, UV resins are prone to yellowing.  This is because when UV light hits resin, it degrades the epoxy polymers, causing "ambering".  There are many factors that determine the extent to which your UV resin will yellow, and some tyers will experience it more than others.
Sometimes my UV Clear Fly Finish flakes off
When UV resin is fully cured but cracks or flakes, the substrate is usually the issue.  There are two main culprits: something with too much give (ex. a loose dubbing ball) or something that is too smooth (ex. some tinsels and waxed thread).  But by far the most common factor in producing flaking resin is waxed thread.  It doesn't allow the resin to fully bond with the thread, and therefore makes the resin susceptible to cracking.