I was present for much of the process. That was mainly because I intended to be present at the end of the job, but it took longer than expected.
The fitter was remarkably erudite, and I found out a great deal about how to do this job, without getting covered in shit. He wore no PPE.
Here are the key points:
Wear rubber gloves (The fitter didn't, but I would)
You can't smell the stench after a couple of minutes, so there's no point holding your breath
You need a large wet and dry vacuum cleaner - preferably one dedicated to this particular job, or at least you'll need to wash it out really thoroughly afterwards
Don't forget to empty the vac, as needed. Into another toilet, or the inspection chamber. The fitter, who knew what he was doing, took a very long time emptying the vac into the other toilet, but he left it pretty clean.
First, disconnect the pan, etc. Vacuuming out the contents, as needed.
Then empty the saniflo with the vac, but leave it in place.
There's a one way valve in the outlet. This holds the stuff in the waste pipe in place, whilst you empty the saniflo.
Then move the valve out of the closed position, and let the waste pipe contents drop into the saniflo. Vacuum it out.
To move the valve out of the closed position, a bent coat hanger would be good. The engineer indicated that he uses his finger, but YMMV. Mine certainly would.
At this stage, you have dealt with all of the really messy stuff.
Installing a new saniflo seems pretty straightforward.
All the connections are done with jubilee clips. Don't overtighten them and strip the threads.
Do make sure that the outlet pipe is really, really well secured.
It's really not that traumatic.
The guy who was doing it has 20 years experience,. Of course, as with anyone who knows exactly what he's doing, he made it look easier than a beginner would find it. However, I don't think it is as hard, or as messy, as is often made out. If you would consider rodding your main drain, then I don't think this is any worse.