The amount of seams isn't the problem, the lack of tape is.

Thank you Joseph, so if he tapes down the seams I should be ok? Also what do you think about the last picture...no plastic or cement board on the beam?

They need to install more fasteners too. Could be that they're going to do that before proceeding with the tiling. But have them do that.

I agree with Joseph, follow the Tile Council of North America guidelines. The would shouldn't be exposed and I'm curious about why there's that wonky piece in the middle of the wall. The Schluter profile will be set on the surface of the board just under the tile, it shouldn't be on the wood. Also, that cement board should be waterproofed with something like KERDI, RedGard, Ardex 8+9 or Hydroban. Do not let them set tile directly to the cement board. And if they say they've done it a million times and had no call backs tell them you want the waterproofing.

thank you so much for your help. I am thinking that he cut the pieces in order to save on the cement board? Should I ask him to take it down and replace with bigger pieces or if he tapes and fills them will that be ok? My second question does he need to fill the taped seams with thinset AND then waterproof the entire walls even though there is plastic behind the cement board? Looking forward to hearing you advice so that I can get them do properly before setting tile

If he has put plastic behind the cement board then you don't want waterproofing on the room side of the wall. Water barriers at only used on one side or the other not both.

That leaves the concern about the 2x4 showing. That wood should be covered with the plastic, and under the cement board as far as I know. He should have shimmed all studs to the same plane and then put plastic over all of it then the cement board.

Remember, I'm a diyer and I could be wrong on the specifics of the process. But I'm not wrong on the principle.

They may not have to take down all the HardieBacker but that back wall is a mess. It only requires two 3x5' panels and should cover that stud. Personally I prefer to use alkali resistant tape on all the seams with thin-set per manufacturer directions. Allow that to set overnight. Then waterproof with something like Ardex 8+9, HydroBan, or RedGard (2 coats in opposite directions). The vapor barrier behind the HardieBacker isn't waterproofing the walls, just preventing vapor transfer into the stud walls.

How I have understood water management with shower walls is that you use either the plastic at the stud then cover with cement board OR apply cement board to the studs then apply moisture management system to the surface of the cement board.

Tape with fiberglass tape and thin set.