Would need to know more info about the OP's garage. In the world of single-ply membranes, TPOs stand out because the early manufacturers latched onto the reflective benefits, which resulted in a lot of architects specifying those systems. TPOs are no better performance-wise then other single-plys like EPDM or PVC. Earlier TPO formulations suffered material failures; the jury is out as far as how long the current forumations will last, although most have been around 10+ years now.
If I had a flat roof a TPO would be one of my last choices, as I'm a big believer in designing roofs for the long term (25+ years). As with any flat roofing system, the success will largely depend on the detailing at the penetrations and terminations. I am leery of the capabilities of residential roofers when it comes to properly installing these types of roofing systems; not that there are not some who can do it properly, but there are many, many more who cannot.