I would begin by asking the tile store if they have anyone who does steam showers.

Steam shower kit? That is not some "kit" you install. It's a custom built shower that meets industry specifications. Your most important decision if you don't want someone to "screw it up" is hiring a qualified tile contractor who's work is in accordance to industry standards.

"Why is the contractor making it sound like it’s the end of the world?"

Because he/she may not be the experienced contractor needed for a proper steam build.

Should not be an issue as long as you are in the design stage. If work has begone and a steamer is a after thought that would be a different story.

Your contractor is not the person for the job you seek.

The steam generator is simple enough, it has water and power in and a drain and steam out. The problem is that you're going to have to have doors that completely close the opening if you want to do it.

Just a few points to ponder, I'm only posting because of the content of the last paragraph of my post.

1) The steam generator is sized to the volume of the space as well as the materials selected in building the shower.

2) Insulation behind the tile can result in a more efficient performing steamer.

3) Waterproofing behind the tiles isn't good enough. You need vapor-proofing as well. Meaning vapor impermeable. There are a few excellent waterproofing membranes that will perform perfectly well in a shower that are not qualified to be used in a steamer. "I always use this waterproofing membrane" is not good enough. "I always use this water and vapor-proofing membrane that has an appropriate perm rating for a residential steam shower" is a move in the right direction. The vapor proofing needs to be on all six sides of the cube. A surface applied vapor-proof membrane will minimize moisture penetration into the wall/floor/ceiling structure. Using a foam backer board the has a vapor-proof membrane on its surface kills two stones with one bird.

4) Slope the ceiling to prevent or at least minimize droplets of cooler condensation from raining down upon you.

5) Don't place the steam outlet where the generated steam can burn you.

6) Everything needs to be sealed for vapor. Doors properly gasketed. Glass-to-tile junctions properly detailed. Vapor-proof light fixtures. Any plumbing penetrations in the walls. Etc.

7) Avoid natural stone and consider only using porcelain tile due to porcelain's low absorption of moisture.

Steamers are fairly straight-forward to build for those that properly plan the build with the proper components and then build according to the plan. It's all in the details. Shortcuts are not allowed. Moisture vapor will find and exploit those shortcuts due to the vapor drive that steamers create.

So many of get screwed up because the builder instead builds a basic shower and adds a steam generator to it. And the people that do that are the ones that usually screw up the simple shower builds.

Last summer a friend referred me to his neighbor. She had paint blistering on the exterior siding on her house, the siding was painted cedar clapboard siding. Pop the paint blisters and they were full of water. On the other side of that wall? A steam shower that had natural marble tile over cement board. The marble had iron content, so it was rusting from within. Orange-brown blotches all over. The stone was pretty ugly. And I love marble. There was no waterproofing anywhere in the walls or ceiling. No vapor proofing. Built during covid, as she was working from home during covid. She used the steamer near daily. The resulting interior gut job for moisture abatement was impressive. It was amazing how saturated the structure of the rooms surrounding that steamer was. On the other side of the steamer was a walk-in closet. Smelled a bit but looked beautiful. Until you took clothes off either the top or bottom hanging rod. The closet wall and the clothes against that wall were covered in mold. You could put your finger against the drywall and poke it though with minimal effort. Move dresser away from one of the bedroom walls and the wall was covered in mold, as was the backside of the dresser, with some mold inside the dresser. Just ugly.