There are people who cheat with plugins, yes. That's a confirmable fact that is very very painfully easy to verify.
So that isn't a question of whether it happens or not, because it does in fact happen and thousands of little yellow school bus riders do it regularly.

That said, I've never actually noticed the people that do this in-game, so I don't think its actually very good for the most part. If you know what you're doing, you can usually outsmart them anyway. Frontlines games almost always come down to formula's. People use the same rotations. You have to both physically look at their attack animations and most importantly, listen to them as well. I'm able to stop most LB's instantly because depending on the job, I can hear when it's starting and interrupt it (some have very small interrupt windows though, so keep that in mind). You also have to generally be able to predict what they're going to do as well, which again, isnt hard when they all do the same thing anyway.

That's because jobs don't actually have that many important skills. Each job is generally going to wait on and use the same maybe 3-4 attacks over and over again. If the best skills aren't up, they run to the back and wait for the cooldown to die. So if it's a drk, right? I know they're going to stall for a bit, and then jump in to pull everyone, do a few attacks with his team, then run to the back again to heal and go back to stalling again. They will not run out again until that cooldown is up, which lets you know ok, by the time I see that drk in the front again, he will again be ready to pull. You gotta put two and two together that way, with all the jobs at once. I say this as the very thing a lot of you hate; The premade player in voice chat. We do the same tricks every time. Notice the pattern, and we're easy to interrupt.

Also as I've said before, it doesnt matter what era of FFXIV it is, Frontlines will always have groups that decimate teams and that's because higher levels of communication. Premades are talking and agreeing on what to do all at once, while the rest of you are just kind of running around aimlessly unsure of what to do and fighting alone despite being with 20 other people. You're not working together, you're not telling each other when to dump attacks, or hide and all ambush. As long as people have friends, there will always be groups that steamroll, because they're actually working together which is the entire point of Frontlines to begin with.