I have seen underfloor heating ---electric and those with water pipes...
1. Is there a disadvantage to underfloor heating?
2. Since it will be placed underneath ceramic tiles or hardwood which is permanent, will future repair be a problem?
3. Since there is heat, will growth of bacteria of molds be faster?
4. What is the brand/model is the best underfloor heating?
A bit late, but perhaps still useful.
I can answer the questions based from an electric heating view point.
There really isn't a disadvantage to radiant heat...about the only thing is that your floor height might get raised by 1/8"...which usually isn't a problem!
As for repair, once this is properly installed there won't be any need to repair, as there is nothing to fail...no moving parts. There's only the heating element under the floor.
Bacteria and molds will not be a problem...just raising the floor temperature some degrees is not enough to do anything...there would still need to be an amount of moisture, not to mention proper oxygen, and so on...and with the heating element itself actually embedded in the SLC or thinset under the tile, there is no oxygen.