Used them in fw for a good while (granted, this was over twenty years ago) but got tired of having to move everything to maintain under them. Like I said, just IMHO, but I'd never bother with one again
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Really, I would just stick the siphon hose down the lift tubes about every3-4 water changes and suck any detritus under the plate out. No need to dismantle everything.
Conversely you can use them in a reverse flow configuration and that greatly reduces the amount of detritus that accumulates in the substrate/under the plate.
In my experience they are more stable than canister or HOB filters and are much more forgiving in terms of inconsistent feeding/bio-load.
You can overwhelm or kill off a canister, HOB, or even a sump system much faster and easier than you can an under-gravel.
The greatest drawback to them is the same as most biological filter systems, they are not that great at breaking down nitrate. Even with all the new tech, nitrates still pose a problem.
Luckily with the increase in knowledge concerning the biological process involved, we have come up with refugiums, de-nitrification filters and de-nitrification media.
As far as being a "nitrate factory", that happens with any type of filter that has the water passing through the media if not properly maintained.