Need to clarify
They do not release nitrates.
They are VERY effective at converting ammonia to nitrite and nitrite to nitrate.
If you had LR rubble in the sump, it would do the same thing.
But if you do not care for either one, do not run prefilters, and they get full of gunk...BOTH will cause potential problems.
The argument against bioballs is that a deep sand bed will "act" preferentially on nitrate produced in nearby surface layers (eg sand surface or LR surface) and less so on nitrate produced "distantly" (bioballs).
But if they are well cared for, they shouldn't be a big issue. They may not be necessary, for sure, but they do not, in and of themselves, cause nitrate issues IMO.
Having LR rubble in the sump is fine as well, but again, if it gets full of gunk and just traps sludge, that would be a problem as well.