Right you are.
I'd just like to add on which set affixes cannot be added by amethyst.
Set affixes that cannot be gained from amethyst crystal:
Defiant, Malestrom,Mayhem,Mashochism, Berserker,Smoke Screen,Epiphany, Seven Deadly Sins, Nadroji, Eternalized, Crystalline, Mythical, Gladiator and Cerebral Vortex.
Apart from the sets I listed above, everything else is available on amethyst. What is in common with these sets is that they are found at high floors, have difficulty requirements sometimes and some of them are rare legends or battle arena only legends. In addition, all are very powerful or very good in some way or another.