Part of what makes Mawdryn Undead such an enjoyable story could also be seen as its biggest detriment, and that quality is this: there is no real villain in this story (other than the Black Guardian, whose appearance here, really, is a subplot to everything that's going on). The main antagonist, for lack of a better word, is Mawdryn and his seven friends, and the only thing they are ultimately after is death.
While an enemy who doesn't want to destroy the Earth or take over the universe is quite refreshing, it does, admittedly, lessen the stakes a bit in Episode Four. However, it is a rare instance that sees The Doctor being the person who is most threatened. Both the Black Guardian (for nefarious reasons) and Mawdryn (for circumstantial reasons) want The Doctor dead. Personally, I think the attention is justified. The Doctor has risked his neck to save countless planets over the last 20 years. He deserves to be the focus of everyone's attention for once.
One aspect of Mawdryn Undead that was a brilliant stroke of genius but was sadly underused is the concept of a story taking place concurrently in two different time lines. It's surprising that, for a programme about time travel, Doctor Who rarely deals with the intricacies of traveling in time, and Mawdryn Undead is one of those few stories that does. It is, however, a highly entertaining story, anchored by two stellar performances from Nicholas Courtney. Inhabiting the role of the Brigadier after seven years away from the programme, Courtney gives us two distinct and believable versions of the Brigadier we know and love - the newly retired, hard edged Brigadier of 1977, and the softer, kinder one from 1983.
And as for the whole UNIT dating controversy that was ignited with this story? First, I'm surprised that supposed continuity expert Ian Levine let the whole 1977 thing slip by in the first place, but I'll say this: wasn't it only ever implied that the Pertwee UNIT stories took place in the near future? Nowhere during that time did anyone actually state what year the stories were taking place in. The only person who lets the side down is Sarah Jane Smith, who blurts out to Lawrence Scarman in Pyramids of Mars that she is from 1980. Personally, I'll believable the straight laced Brigadier more when I want to know the date, as opposed to that flight risk of a supposed journalist that Sarah Jane Smith is.