Unless you had dozens of different types of TV's with a controlled test, it's hard to really quantify how good that RF modulator was (or how bad). Also, you said you don't remember what the third computer was. Are you sure you remember it having a better display?
When I was a small child, I had a TI-99/4A and a Commodore 64C. Both of them were RF only. Maybe I was lucky enough to have a good CRT or maybe I didn't know what composite was to compare it to. But I don't recall any one being better than the other.
I now own many computers in my collection and I have a few RF modulators from TI. On one of my CRT's (some generic no-name), even RF looks good. In fact, my NES top-loader looks fantastic on that TV using RF.
So it's quite possible that you just had a CRT that didn't play well with that RF modulator.
Also, the Colecovision used the same VDP (TMS9918 in the US). I had that as a child (with the cheap RF adapter it came with) and I still don't remember it being any better or worse than the TI or Commodore.
Finally, I have an Atari 400/800/600XL/800XL/1200XL/65XE/130XE/XEGS in my collection. Just about every 8-bit Atari there is. And every one of them look like garbage on a DIFFERENT CRT I have.
I put an S-Video mod on my 800XL (super easy) and my 800 already has S-Video (separate chroma/luma). When I utilize the S-Video's on those (or even composite for that matter), it's night-and-day difference. Even though I know the Atari has good video output, the RF for MY particular setup looks bad.
So it really is several factors:
What kind of local interference do you have? You mentioned three TV's right next to each other.
What kind of cabling/shielding do you have?
What kind of CRT/RF modulator combo do you have? Some play nicer than others.
There are SO many variables that to say all TI's had terrible RF output is simply not correct. My experiences were much different than yours using the same computer.
Keep in mind, too, that the "R" in RF is for Radio. Literally any sort of radio interference around you can cause issues and noise.