Depends on the qualifications of the installer, the quality of the stone and the understanding that you have (and willingness) to maintain it. Been installing natural stone in steam showers for eons with no issues but these clients could afford quality product and qualified installation and were all in on the maintenance. (VENTILATION, wipe down after use, proper products for cleaning, leave the door OPEN when not in use, weekly cleaning,...). "Tile experts"? Mold and moisture issues are true for any material, even ceramic, if not maintained properly.

We are putting in a steam shower and were told by plumbing store, tile store, etc. it was best not to use natural stone, so we picked something else. I believe if you use natural stone you may also need to increase the size of your steam generator (depending on the size of your shower and where it falls in the cubic feet recommendation). The steam generator we’re going with has an online selection tool that takes material into account. Good luck with your remodel!

It is best not to use porous anything in a steam shower. The typical lowball consumer has a 1 in 10,000 chance of getting someone that actually knows what they are doing to build most steamers. They won’t pay the substantial additional money for what it takes to hire an expert. So they build in problems on the front end. Sticking with porcelain at leaves gives a few years before it falls apart.

"it was best not to use natural stone, so we picked something else. I believe if you use natural stone you may also need to increase the size of your steam generator "

Among many other reaons but YES somebody is paying attention. You are very correct and clearly working with smart builders

Is it okay to go with an engineered marble vanity top in a small bath with a steam shower?!