It all depends how mechanics are tailored around that. If you play WoW or Rift, despite having a UI that can be highly customized, end-game encounters remain harder than anything XIV has implemented at the moment.
It's easy to see if an UI is game breaking or not. You just check the average time needed for a game population to defeat latest implemented raids and you check how fast people progress through raids. In XIV, in a matter of hours/days, the newly implemented content is cleared by "top" players. Quite soon after, a good chunk of the population is able to clear anything that's implemented. Especially if you use JP players are reference (less affected by latency).
Now, if I take Rift as an example, latest 20mans raid was implemented on April 18th. The Raid have 8 bosses.
First World kill of boss #4: April 22nd (all boss before that one went down "quickly")
First World kill of boss #5: May 25th
First World kill of last boss: June 10th
So it took about 2 months to get the instance cleared by top players in the world (and they are hardcore players :x). Even as of today, end of August, only a small amount of guilds are past the 5th boss. And a very low amount of guilds defeated the last boss. We're 4 months past release and people are still progressing through this raid. And that is with UI mods, parsers, you name it...
Also please note that unlike XIV, where every new killing attempt is a hassle and often mean high downtime, in this game, you are good to go in ~5 min after an entire raid wipe. Anyone can just AoE raise, people rebuffs, and you go again. And despite all that, we are still looking at months to clear a raid.
To understand that such UI element aren't game breaking if mechanics are done properly, you must have raiding experience in another game could it be WoW, Rift or similar. If you have no raiding experience, then you cannot understand how these tools doesn't "play" for you. Skill is more involved than ever.
Giving lot of tools to players is counterbalanced by having lot of mechanics put in place. It's like driving a car vs piloting a huge airplane. Yes I do not need 545687 assists to drive a car, there isn't much to look for. But however I do need tons of electronic equipment, assists, automatic things when piloting an airplane. In the end, which one is the most complicated? Driving a car? Or piloting the airplane?
Then if DEVs are stupid and allows lot of UI mods while gameplay remain as it is now, obviously it will kill the game, yes. Driving a car with automatic pilot isn't challenging.