I've heard that the BBC decided The Honourable Schoolboy was too complicated to dramatise, after the success of Tinker, Tailor, Soldier, Spy. I think it is a more difficult work, but I suspect the reasons are more proasic - both the TV and the later film adaptations shift the initial action away from Hong Kong to the Mediterranean, which obviously jars with continuity of some minor aspects of the storyline here. Furthermore, the TV adaptation cast Joss Ackland as Jerry Westerby in Tinker, Tailor, and that just doesn't seem a good fit for the character here.
It was right for me to pause between books, but not to pause too long, because various threads do reappear here, and it certainly can be difficult to keep track of all that's going on. There's a lot of Circus politics as well as the espionage story: following "the Fall", and Smiley's reinstatement and appointment as caretaker head of the Circus, there is a lot of exploration of what worthy material they have left. They discover some hints (suppressed by the mole Gerald) of a Karla operation in Hong Kong, and Circus "irregular" Westerby is sent to probe, under journalistic cover.
Perhaps this is an area where the novel becomes less successful, as jumping around south-east Asia on the trail of dead pilots and drug smugglers does become less clear to follow, especially when intertwined with not only the East-West Cold War but also Sino-Soviet relations.
Most perplexing, perhaps, is the conclusion. Westerby goes off piste over "the girl" in James Bond fashion, his actions escalating to threaten the whole operation; no wonder he's taken out by his own side at the end. Meanwhile, Smiley becomes sidelined as the Americans take the reward, and a new head is installed at the Circus. I had always assumed Westerby was the Honourable Schoolboy of the title, but both words can be interpreted in a way to make them describe Smiley instead - a fitting ambiguity for an author for whom things are rarely black and white.