The main problem with GCD macros is lag. Inherent lag in the macro. I'll try and take a video tonight to show you the difference between what I mean here. With a skill on the bar, you can queue it up. You can hit the button before it's fully filled and it'll activate as soon as it's done. Pressing it sorta has a 1-second delay during which the skill is in queue and will be used if it becomes available. When you use the /ac command, the action is activated immediately, unless it's not ready, in which case it's dropped from queue immediately. It doesn't have the same grace period. This causes inherent lag in skills because you have to press the button immediately as the GCD resets, the game then has tor register the button press, and then the skill activates. Compared to a skill being on the bar itself, where you can tap it once within that half-second or so grace period and it'll activate the moment the gcd finishes refilling.
That's not to say you CAN'T use them, I just personally think they should be avoided whenever possible. Life Surge > Full Thrust mostly causes issues if Life Surge comes back just as your GCD resets and you're up to Full Thrust in the combo (which *does* happen, every time you do the full rotation proper) because your macro will force you to activate Life Surge and delay your GCD despite it being full. Without the macro, you'd just hit FT as LS refills, tap LS, and hit FaC/WT. With the macro, you'd spam your FT macro with full GCD for a second while LS animates so you can crit FT before you hit FaC/WT and possibly loose Blood due to delay that shouldn't have happened.