From the FM transmitters I have tried, they will typically have lower HF response, lower LF response, lower stereo separation and may also have bass weirdness if the LF filter is badly implemented. The quality of transmitters also varies greatly, but it's hard to find ones that can match the quality of a commercial radio station, at least I haven't come across one that was convenient to use in a car.
That being said, your ability to tolerate these differences may different from other people. So if you have the ability to try it and it sounds fine to you, go for your life.
In any case, if you have the ability to plug in an AptX receiver - for me these have been substantially better as a wireless connection than FM.
First, the FCC specs for FM are minimums, not maximums. The top end is limited by the presences of a 19kHz "pilot" tone at -20dB re: 100% modulation, and is required to demodulate stereo. You don't want audio around that, but modern broadcast transmitters can be flat to 17kHz, and, in the old analog days, included phase compensated low pass filters to protect the pilot. Today the entire FM signal is synthesized digitally.
The low end is (was) limited only by the FM transmitter's automatic frequency control, which wants to "lock" the carrier to a particular frequency, but still permit audio modulation. Broadcast AFC circuits are complex, two-stage PLLs (if analog at all), but permit full undistorted modulation down to 20Hz. Broadcast transmitter stereo separation is better than 40dB, often as high as 47dB. That's transmitted, but a receiver may not realize that potential. The transmitted FM signal from a broadcast station is always way better than any receiver can produce.
The problem is doing any of this cheaply. Just not going to happen. There must be compromises in every aspect. What's worse is, our little FM transmitters appear on our car radio dials right next to the real stations. Real stations use thousands of dollars worth of audio processing to the the average levels up and loud without shooting over the 100% FCC limits. That's not an easy task, hence the expense. Again, you can't do that cheaply, not even close. So our average levels on our little transmitters have to be kept lower, because FM radios have less than 3dB of headroom above 100% modulation. Without some serious processing, we can't hit high averages. That means our audio will be lower than broadcast stations, and with the lower transmitted power, often noisier. It's kind of a no-win, really.
But the fact that it works at all for $50 or so is miraculous.
