What disadvantages do thermostatic mixing valves have?
I would say that the advantages are number one,  it’s comfort, you’re not constantly going back and forth trying to adjusts to get the right temperature and in the process you probably use less hot water and that being the case it probably saves you some money on energy.  Also the main disadvantage, if there’s a problem with it, it’s pretty expensive labor wise and material wise to dig them out from the wall and repair them.