personally, I enjoyed UA. It's a much darker book than some, as is Night Watch .. but then again, I enjoyed NW too. I also enjoyed Reaper Man very much.
I'd take the view that the best writing can span various moods. Dad's Army is silly slapstick on one level but very well rounded; Mainwaring is a blustering incompetent and a petty snob, but he is genuinely brave ( if misguided ) when threatened, for example.
There are some very good running jokes, such as the question of Frank Pike's paternity, and the Jones and Mrs Fox wedding is a good aaawwww........... ending, especially with the revelation of Wilson's past as a decorated combat officer in the Great War. The episode involving Wilson't abortive promotion is a slice of straight drama, really quite poignant and with virtually no other characters involved.
My benchmark for this kind of writing is Evelyn Waugh's Sword of Honour trilogy, combining the farcical slapstick of Apthorpe's thunderbox or Tommy Blackhouse and Guy Crouchback's dinner with the Laird of Mugg, with the bleak decline and fall of Ritchie-Hook, Trimmer's absurd infatuation with the ruthless and self-centred Virginia, the surreal machinations at HOOHQ and the subtly interwoven plots by which Guy Crouchback sails through life by no real effort or design on his own part.
Waugh also does sex, which TP doesn't; but in a very discreet way. The scene of Guy's failed attempt to seduce Virginia at Claridge's is a masterpiece of sexual tension leading to a quite abrupt disaster, when both say things which cannot be unsaid.
my personal view would be that TP is on a par with P G Wodehouse but has the same limitations, of willingness to sacrifice the plot for a joke and undue reliance at times on deus ex machina plots.
I do hope he doesn't develop Niven's Syndrome, by which once-brilliant ideas are compromised beyond repair by endless attempts to connect things which were originally quite unrelated, or explore endless variations of a basic idea which make no useful contribution to the plot.