No downside really. Bigger tank owners with healthy SPS loads typically feed heavy and export heavy, and the formula seems to work.
Where I don't run them is when low nutrients are a problem. "Just feed more" isn't an answer. I have a 20L growing SPS like wildfire and I had to take my skimmer offline to keep nitrates and phosphates from bottoming out.
DOCs are of course an issue, but I run ozone a few hours a week and that thing does in two hours what my skimmer can't do running 24/7.
I don't agree with oxygenation being significant because the amount of surface area of a tank vs air bubbles is insignificant. This has been beat to death in FW forums. The typical flow rates we run in reef tanks cause orders of magnitude more gas exchange at the surface than a bubbling skimmer.