You were right: supernatural abilities, flame arch Included, do not provoke opportunity attacks. Nor can they be countered, dissipated or altered. They never require concentration controls. Basically, antimagic field It's almost the only thing that can stop them.
This makes it similar to, say, a dragon breath weapon. These are also supernatural, and often form lines of damage. (None seems to breathe a line of fire, but that's irrelevant.) Dragons certainly do not cause you to breathe.
But you say that flame arch instead, "acts like a bolt of lightning", which could not be further from the truth. Flame Arch 1d6 damage done by two levels: Thunderbolt it is divided twice (or more, in odd levels). Flame Arch adds the Arcanist's Charisma bonus, which may be substantial, but is unlikely to compensate for many days. That means that a shift spent using flame arch– assumingone there are no resistances or vulnerabilities at stake; considerably less than a turn of use Thunderbolt. What very often means it should not be used at all.
The issue is that the Pathfinder bouts do not involve many turns, and many things happen in each turn. In the middle of a fight, actions are the most valuable resource available: save a spell slot or what does not matter to you if you are dead. Y flame arch It's not free: arcane reservoir points are being spent. Yes, it's a bit safer to use than a real spell, but that's rarely a major problem since most spell casters 1. five feet away from threatening creatures, and 2. throw themselves into the defensive when they can not.
And finally, it must be said: Thunderbolt It's a poor spell. Blasting spells, in general, are quite poor in Pathfinder. Their damage is difficult to improve and does not do enough to be worth it. By default, they are worse than attacks, assuming that the decently selected dowries are improving them. So comparing flame arch Against it, it may not be the best example. It says enough that flame arch it does not come out of that comparison too hot.
- If we do not assume that we can ignore resistances and vulnerabilities, things only get worse for flame arch in comparison with ThunderboltResistance to fire is found in more monsters than electrical resistances, and in most campaigns will be more common.