Here in NL had to install the transponders already. Therefore we have some experience with them in terms of failure rate.
I do not own any transponder, but I heard from a reseller the following "ranking", from more reliable to less reliable:
1. Trig
2. Garrecht
3. Becker
4. Funkwerk/Filser

A nice feature of some units is a split accesspanel-radio unit.
You can then install the panel in front and the radio unit behind you, thus reducing long antenna cabling (and Rf crosstalk with other electronic wiring, like the radio).