They don't have to take up loads of space though, you can lift the blade up out of the way and turn the whole thing 90 degrees so it's at the back of the bench out of the way. They do need more depth to the worktop to cut at full width and that's compounded by the fence being relatively further forward, but you can counter this by building it into a bench with a drop-in section so the deeper bit can be removed and swapped for a normal depth bit of work bench.
When installed as above it'll take up less room than the SCMS that can't lift and swing itself out of the way. Easier to move the SCMS into a cupboard etc, but most people will just leave them out on the workbench.
I bought one ages ago because to get the same capacity, thickness and width of cut with a SCMS would have cost a small fortune. I'd say that unless you get a really good one, you need to factor in the time to make a decent bench that's accurate and to strip, clean, rebuild and replace bearings etc if you want to get the same accuracy as the SCMS is capable of. They're old kit and just aren't as well designed in terms of being able to quickly and easily setup accurately if you need to change between angles etc regularly. This is especially true if you want to cut trenches as you'll need to be able to adjust the table bed to be exactly parallel with the saw blade path.
Also, you should really fit a negative rake blade to stop the saw trying to accelerate into the cut (and your fingers!), which are tricky to find and you'll want a blade with flat top tooth profiles for trenching cuts or you'll be having to tidy them up on the router table.
Dust extraction is rubbish, so you'll probably want to look at building some sort of hood for that, though my SCMS needs a hood too as the extraction on that isn't very good either.
