The issue is this: they're aerobic. Sole function is ammonia filtration, these filters add oxygen competing bac to a system that runs fine without them. Your ammonia is zero with or without them
Why use them in reefing, then? Sole target of all high surface area aerobic filters is ammonia, solely. Your tank handles all ammonia the same, without them, and in a power outage these become a concentrated liability.
Oxygenation=same as above, not running low on o2, and you're adding oxygen competing bac that take it from your system.
The only time these filters are used accurately is in the presence of such bioload that free ammonia results without them
Additionally, using them as GFO or biopellet reactors or carbon filtration is legit, since the target isn't ammonia which was fine when cycling completed with or without extra surface area added after. Carbon filtration for water clarity/legit use. Must keep clean of detritus to retain benefits.
They’re totally unharmful to use. Any tank here can connect three extra canister filters and it will not harm anything at all. Measurable params remain same with zero or with three running
*the fact you can instantly remove an aerobic filter (or sandbed

) immediately in a reef tank and nothing is lacking regarding filtration surface area is a neat way to measure their redundancy.