I have not used this product but the underlaying technologies are very familiar.
The microphone picks up “all sound in the room”, better for closer sounds. Great if you get close to a dominant sound. Not great if there is a jackhammer around (or at the zoo). If the best position for the mike is not near your ear, we need to carry it there “somehow”.
The T-coil picks up all electrical “noise” in the room, better for closer sources. You put the listening T-coil next to a plain telephone, or a sending T-coil, and it hears that best; but it always picks-up buzz and static from all the electric power and appliances in most rooms. The stand-out advantage is that “ALL” aids can have T-coil… it dates to the first half of the 20th century.
FM works same-as FM broadcast radio. It tunes to its assigned channel and rejects most of the other stuff in the radio air-waves. The modulation is very specific and the FM receiver rejects most buzz and static from non-FM sources. FM is a mature and fairly easy system (now a small part of a small chip).
The new way is BlueTooth. But this is a more complicated protocol and was developed for larger machines (laptop PCs, mice). For straight sound, FM may be better, certainly cheaper on the chip and in battery life. BT allows more complicated data error-free, but older implementations were power-hungry. That’s got better.
If the T-coil is not picking up much crap from airwaves (and has enough bass for your needs) and enough range), it sure is simple. FM will have more electric air-crap rejection. BT can do much more than this job needs.