Do these not work very well?
Depends, Some do Some don't.
the pipe that comes from the separator over the top of the engine and into the turbo inlet pipework has a couple of different types.
One is just a standard pipe with the same internal diameter all the way through (this is what mine had) some have a pipe which is the same external diameter but the internal reduces the closer it gets to the turbo pipes.
Apparently this was to restrict the flow through the separator and give the oil more time to condense and return to the sump.
Sometimes this wasn't enough and you still got a lot of oil going back through the turbo.
Some people fit additional oil catch tank and block the hole into the turbo pipework to keep the inlet air cleaner and less oil contaminated.
This can improve performance (slightly) and 'might' reduce the crap that gets into your DPF and inlet manifold.
I had a catch tank fitted for a while as my 1.9 seemed to blow a lot of oil through the separator and into the inlet.
Some cars do, some don't, just one of those things.