They are probably referring to the fact that you are not cycling through your targets and using different sections of your RoH combo to maintain enmity instead of using Flash for your ancillary targets.

Using Flash for the 2/3/4 targets is fine, but it also means that you do not have your pulse on the battle, meaning you are not able to keep a keen eye on each other target - maybe the archer hit a target with a DoT when you weren't expecting it, or the WHM had an extra big crit heal and you are about to lose aggro on the 2 other targets. Basically, if comes to down efficiency and providing a good cushion on each of your targets, not just a huge cushion on one.

But if you aren't losing enmity, you're fine : D