What you're describing isn't a raid buff issue. It's an issue of sustained damage output vs. burst, and it occurs even if you only have access to personal damage buffs.
Sustain and burst exist on a continuum. No job is truly 'sustain-only', because that means that all GCDs are equally weighted to deal the same amount of damage. Such a system would have no decision-making, because all actions have the same effect. It's better to think of each job as having a 'burst profile' that weights more of the total damage output on fewer GCDs. You haven't really changed the area under the dps curve, just the distribution. Some jobs just have more burst than others.
The problem with having large variations in burst profiles is that it becomes harder to tune job performance on a fight-specific basis in a way that's fair. If the jobs are balanced for a target dummy fight with full uptime, then a timer-driven burst job will pull ahead the instant that you have intermission phases where you can't hit the boss. Your timer still ticks down while you're in intermission, so a greater proportion of the 'active' fight is spent in burst. This sustain disadvantage was a big complaint about DRK during Stormblood, which is why it's moved to the other extreme over two expansions. The PLD changes this expansion boil down to the same issue, but it was likely exacerbated by how intermission-focused P8S P2 was.
The bottom line is that large discrepancies in burst profiles can result in performance differences within individual fights, especially if they have intermission phases. This creates a selection pressure for all jobs to become more burst-focused over time. Resource-based burst can be a mitigating factor in this, because you're not gaining resources if you can't hit the boss. But resource-based jobs also have an edge if there's an external buff that you can pool into (i.e. Everburn). It's actually quite difficult to balance fairly under different conditions unless you impose some constraints, either to the fight design or job design. Some variety is good, but when there are large differences you start to run into problems.