You are inferring that the purpose of the purify recast reduction is to negate the smn burst. Obviously it will indirectly nerf it somewhat when its used before their fester, but no its to counteract the increased amount of dots that are flying around now from the influx of smns stemming from the introduction of tri-disaster, plus the fact that unlike the other side of the spectrum with medica 2 and regen these smns dots all stack with multiple smns.
Besides, fester is still an important part of a smns burst despite you downplaying it and their effectiveness is dependent on these dots. This makes smn the only job that can have one of its burst abilities completely negated by all jobs with the use of just one ability. I dont know what to tell you if its taking 1 to 2 aetherflow abilities to be able to purify the dots off other than reassure you that yes it is indeed possible to do it before fester hits as im hit with festers that do zero dmg in every match i play and i know im not the only one.
As strong as painflare and deathflare are they still arent strong enough to 2 shot you and enkindle has like a 3 minute recast. If they have access to and land all these abilities at once then of course its gonna kill you which is the point of a burst is it not? And the penalty they are suffering are the 60 seconds of complete uselessness they experience after expending all their stacks since smn short term dmg after a burst takes the biggest plunge out of all the dps.
The biggest issue is indeed the fact that they can use their burst at a distance but if you do something like require them to be within melee range to perform the burst then you may as well not let them burst at all cause they will be murdered b4 they can even get halfway through their burst. Leaving only really the option of nerfing some of the individual burst ability potencies. The battle classes in this game arent balanced for pvp regardless though and there will always be people that can exploit that by switching to whatever is the deadliest job available when played at max efficiency.