The biggest problem of FFlogs compare to Warcraftlogs is that there is no split by item level. So if you have two players with 350 ilvl and 370 ilvl and same skill, guess who ll have higher percentile.
The other problem is food/ pots are required if you want to push for top percentile as well.
The raid composition matters as well, for example, BRD without DRG buff and with DRG buff have completely different percentile.
Overall, the correct rotation and the uptime contribute the most to the percentile, but there is no need to be obsessed with purple/ orange parces as long as you can clear the encounter.