It’s completely possible to melee bosses in this game and fractals. I’ve done it with my guard into the 20’s of fractals w/o dying. Granted you can’t melee 100% of the time. There are only 2 bosses in fractals that I can think of tht I don’t melee: final dredge bosses and ascalon final boss.
That said, generally I think it is harder to melee bosses. However, there are a few bosses that I can think of (TA vine boss) that are easier to melee.
Nanuuk (80 Mes), Guardian Nuuk (80 Guard),
Warrior Nuuk (80 War)
[ALS], Anvil Rock