Their popularity was because of aggressive marketing and sponsoring from USR, rather than being technically better than the competition. In the 1990s loads of Swedish BBSes had small USR advertisement blurbs on their login screens as thanks for the discounted hardware. The push even made it all the way into the elite boards (invite-only warez BBSes). Source: I ran two BBSes in the 90s and spent a few years co-sysoping a large elite board along with doing a lot of "modem trading" (slinging warez between boards).
Could be. I ran into a number of BBSes that dropped subtle hints that they hosted stuff that was not accessible to regular users. I always had the impression that it was adult content, but it could very well have been pirated software. (These were publicly listed BBSes.)
Most pirate BBSes publicly advertised themselves, as a form of bluster.. But having the phone number and getting an account were two very different things. :-)
You had a Courier or nothing. They were amazing if your ISP (or warez bbs) had them at the other end. Proprietary compression / connection algorithms, I guess?
Pretty much the only modem I got ACTUAL 56K speeds with was a Courier V.34 that I flashed to become a V.90
The standardized compression is called v.42Bis, but it was in fact a feature present only on few of USR's models while most of the other brands provided it with consistency. It worked well on plain-text content - I remember hitting 90 KB/sec download speeds when testing large text files on a 28k8 connection - and it made a noticable improvement when bouncing between menus and reading messages and such, but very limited use for actual file transfers since practically everything was already lha/lzh/lzx/dms-compressed.
Or at least, many pirate boards only accepted those connections from other USR modems.