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I think the biggest thing is be prepared to strip more back than you expect... I had not anticipated going back to joists to get all the rotten/mouldy wood out. I hear this sort of surprise isn’t uncommon.
Other than that it’s lots of small quite specific things - weird waste pipe sizes, corroded brass fittings that refuse to re-seal properly, tiling round the edges being a total PITA when done after the middle, getting the right sealant for the job, etc
Edit: oh and if you’re having a walk in shower with flappy glass panel, go longer on the main panel than you think (and work out where the shower head will be).
In my limited experience (one en-suite sans toilet), most of it is easy but not necessarily immediately. That’s ok when it’s reversible or a small job that repeats but not ideal when you’ve only got one chance to get it right and mistakes will be expensive or very obvious. Some of it is a lot easier with two people.
I read a lot, got a lot of advice on here and paid for labour on a couple of jobs that would have required excessive outlay on tools and experience to get right!
I also made it in sketchup first to work out the order I needed to build it in.