why do some RC models hace the horizontal stab on the bottom of the fuselage like the Ugly Stick? What are the advantages and disadvantages over having it on top of the fuse?

For most sport models, it’s what the designer though looked good. In your example of Ugly Stik, it was done for simplicity. When looking at models that have been specifically designed for precision aerobatics, the stabilizers location greatly contributes towards the elimination of both pitch and roll couple with rudder application.

Speedracer covered 99% of it. There is also ease of setting up the control linkage though that still falls under simplicity. Related to the comment about precision aerobatics there are also some specialized models where the tail surfaces are placed so that they will see non turbulent air for purposes such as maintaining control during STOL flight.

the High v. Mid v. Low stab has yaw-to-pitch coupling implications...

generally, a high stab will pitch pitch down with rudder input, and a low stab will pitch up... so diligent designers will seek the Goldilocks stab location... if you look at current IMAC designs, none of them have a scale stab location... (Usually lower)

tufting video:

Have you seen all the Quique Somenzini model see were the STAB is placed... at the lower side and never on the upper line of the thrust line.

I’m actually in the process of updating my pattern plane that is of my own design. One of the revisions is raising the stab location to eliminate a down elevator mix with rudder application.