I'm obviously biased but I'd say he is underrated especially compared to a lot of his British counterparts. He often gets overlooked as an ATG British fighter (and is the best ever Scottish fighter imo), in favour of flashier fighters with bigger personalities like the Calzaghes, Hattons, and Naz, all of whose bodies' of work is either equal with or obviously surpassing. He's a top 20 135lber, which is no mean feat considering the depth of the division. He squandered a lot of opportunities with his bitterness and mismanagement, along with some misfortune with refs and judges.
It's a tricky question. Breaking it down: He generally gets a fair deal from within the sport. He's a justified Hall of Famer, and widely regarded as one of the elite post-war British fighters. The loss to Duran, if not having an asterisk against the W, carries the caveat and context of the low blow and the lack of a rematch. The manner of his loss is often brought up, which tends to reflect better on Buchanan than if he had suffered a more conventional stoppage loss. Not necessarily that Buchanan was going to win (he almost certainly wasn't), but there is a sense of unfinished business with the Duran-Buchanan story as he never got back in the ring with his conqueror, and the last image of them fighting is Buchanan writhing in agony after getting punched square in the bollocks. In terms of the general public in the UK, he is often overlooked in favour of flashier, more charismatic box office fighters like Benn, Eubank, Hamed etc. He doesn't get his due in terms of recognition, but that is more a reflection of what casual fans look for and how they engage with the sport. Nothing wrong with it, but they think more in terms of highlights and events, rather than resumes. I think the other factor is that he had the misfortune of running into the possible #1 all-time in his weight class. Without Duran, he feasibly could have had a lengthy reign and put up impressive numbers for title defences. In all-time lists you might see him lower reaches of top 20. With a double digit title reign he could be several places higher. You could run "what if" scenarios for all fighters, but it's an obvious one in Buchanan's case. Most fighters don't have to deal with a Roberto Duran emerging in the same weight class during their prime.