There are no downsides to using it, it’s just Smart Fuzz.
There may be a downside. When I tried using the load balance feature in the FSRS Helper add-on, I started seeing related cards (cards related to the same topic but which were not siblings) on the same day more frequently.
My explanation for this was that cards were being pulled from adjacent days into days with low review load, increasing the chances that related cards will be shown on the same day.
I turned off the feature and the issue disappeared. I am not very sure if the issue was caused by load balancing or it was something that happened by chance. But, there is a possibility that something of this sort may happen and people may want to disable load balance.