Another key reason for its benign profile is that it is in a class of antibiotics least likely to encourager resistant strains. Some people take it for decades. Others for just a few months. Some people take them indefinitely. It's a case of early treatment equals in many cases early remission.
Later treatment takes longer because the bug is more entrenched.
This is primarily for RA, but there have been recents indications that it works to a lesser degree, probably less as an antibiotic than as a disease modifier, for osteo. As to allergies, most of them are formed in the surface of the body especially the skin. The way around this is often IV therapy, which works extremely well even in cases where there is no allergic reaction. Many more are using it than just a year ago, and the MIRA trial and another early study met the FDA requirement for definitive evidence. My own opinion is that the biggest barrier to the use of this therapy by rheumatologists is that it kills the goose that lays the golden eggs.