Well, it is interruptible; I've stunned plenty of melee lb's unless they pop fetterward before hand but even then I've canceled it just by immediately purifying and walking away. Honestly, I find it's easier to break a melee lb than it is to purify tridisaster before you get blasted by fester (timing is pretty close I think, but melee lb just has more telegraph to it for me). I think it is pretty tough to stop personally, but since limit breaks are pretty rare in feast, I'm ok with how it is overall.

And are you sure the caster lb breaks from regular attacks? I feel like the healer lb isn't interrupted by basic attacks. I haven't really tested it myself but it's hard to imagine that all the times I decided to use the healer lb, people would suddenly stop attacking me. I've only ever stopped lb's with cc really ( or moving out of range in the melee lb case).