I've looked into these in the past but ultimately decided against them for a few reasons.
1) Can't see inside. Sounds dumb, but I want to see inside my filter chambers to identify cleanliness and any potential issues. Also, I just want to be able to see what I'm doing.
2) Space, they just require a lot of space and I feel don't work well for smaller tanks under 6' due to the amount of space required, even more so if you need a skimmer chamber.
3) Height, I feel like anything that tall will become a pain to work around. Going back to #1, if I can't see in from the side that means I have to have my head under the stand looking down into it, which just doesn't sound very appealing even IF my big head can fit reasonably between the sump and stand. I shudder to think how difficult it would be to clean out taking into account the height and not being able to see through the sides.
4) Filter Socks only. That's a no go for me, personally I prefer to use filter floss laid flat.
With all that being said, Its not a bad system at all, just my preferences keep me from wanting to use it for my use case. I think this system is much more popular among freshwater hobbyists on larger tanks.