wet room downstairs only

No Not at all!
I have installed many wet rooms in upstairs locations.
The secret behind a good wet room is the time taken to prepare and seal or "tank" the room to ensure that no water leakage will occur once the install is complete.
There is a wide range of materials and systems on the market to allow you to create a wet room out of your existing bathroom. The most important aspect is to ensure that the installer has good experience of such installations, as they can be quite expensive to install and even more expensive if they leak!
I prefer to use Schluter systems when i am installing a wet room. This is my preferred supplier, but many more systems are available.
My advice to you when installing a wet room is Don't try to cut corners on the materials, they are worth every penny, and do research into finding a competent installer that can give you references.

You can install one anywhere, if you have been told only downstairs don't use this plumber as he does not trust his work. Fitted 100s of wet rooms last one was in a £50M mansion in Knightsbridge. Beware of Plumbers from London, if he comes after 3 in the afternoon he is not from London as they are all in the Pub at this time checking their stocks and shares. Make appointment after 4 this will ensure he is a real plumber.

no, can be done upstairs

It's not the case you can make a wet room upstairs it needs to be completely water tight by adding an membrane and doing the job correctly. What people don't understand is bathroom needs maintainance because of steam,water some people attend to ignore the first little damage they see, but not relising moist is making it worser. once it start to leak downstairs two years later that's when alarm bell starts ringing but it's to late then. I hope this helped

No you can fit a impey deck to joists upstairs , hope this helps

definately not installed wetrooms in various homes upstairs, the dipped shower area takes the volume of water not the full floor so floods are the thing of the past

In my opinion that's ridicules you can have a wet room in any floor you just need to use the right material.

Hello , could you explain exactly what's the issue !

Thanks

No