That is interesting JohnJay, as builder told us since its a new houes, a3 rating, it's super warm and that will cause black mold to form quicker than seen in older houses.
Mold grows in spots where condensation forms. Condensation forms as a result of warm moisture laden air coming in contact with a colder surface. If your house is super warm, you won't have too many cold surfaces! If your ventilation is up to scratch, you should be able to keep humidity under control. I'd be worried about a builder giving you that kind of information now.
I have found a lot of material online about it... but i don't know who to trust.
I go on to one website and it states you can't use hardwood, the next says you can.
You can certainly use hardwood flooring with underfloor heating. It will act as an insulator as others have said, so it will cost you extra to run. You need to ensure your underfloor heating system can be set to run hotter in the rooms with underfloor heating to compensate. One example calculation from Junckers states that the underfloor heating needs to run at 37.5 degrees in order to achieve a surface temperature of 27 at the floor. See their technical guidance doc [broken link removed].
What is very important is the quality of the product used, and the skills and knowledge of those laying it. With the underfloor heating, there will be significantly more movement in your floor, so all joints will need to accommodate this movement, and you need to be happy that there will be small gaps between boards as they contract.
Engineered hardwood flooring will be a lot more stable.
One independent (seemingly) said that wood could be used but had to be very cautious to prevent moisture damage. i.e. let the subfloor and wood acclimatise over a few weeks.
Again, any product suitable for use will have guidelines such as the Junckers one above that will state what levels the relative humidity of the building and the flooring need to be at before installation. If the shop can't give you a copy of those, go elsewhere.
You will need to leave the new floors, and the rest of the build to dry completely, again ensuring the RH values are at or below the manufacturers guidelines, this can take weeks or longer in a new house. Then the flooring will need to acclimatise for a time in the room in which it is to be laid. After installation, you will not be able to use the underfloor heating for 2+ weeks, and then it will need to be introduced gradually over a week or so before hitting full temperatures. If the prep isn't done right, you'll end up with a mess that will only be suitable for a skip. So perhaps consider a temporary floor if you can't comfortably hit the humidity levels now and go without heating as required.