When my 20/80 cataract-impacted vision immediately spots the seams, like Hesh Rabkin in the Sopranos asked for an opinion, I'd venture "not good".

As above. This is not good. Not the worst I've seen but far from correct. Stop the install and get him/her to take it up and start again.

Now to the technical. Did the guy do a flood test? Are they adding water proofing to the walls? Do you have in progress photos?

We've learned that when a tile install is bad, there are more problems underneath.

I’d suspect it all was done improperly. Gut and redo most likely.

Every time I think I've seen it all here, I'm proven wrong. Red Guard in spots for the win.

I'm assuming the spots cover the fasteners. If so, he is woefully spot shy. Durock requires fasteners every 8", 6" on the ceiling. Page 3.

Your time here on houzz is best spent ASKING how to HIRE the right & qualified tile contractor, where to source a tile pro, how much you can expect to spend when engaging with A tile Pro. how to read estimates, how to protect yourself with written contracts.........

Youve got a full tearout on your hands. #tilefacts

Looks like the installer didn't bother reading the backerboard manufacturer's Installation instructions. For instance:

I suspect if the OP had questioned the "expert", their answer would have been, "This is the way we've always done it. No problems!"