There's a besetting difficulty in adapting the Sherlock Holmes stories for the stage or screen, which has never to my mind been satisfactorily addressed. The playgoer or tellywatcher, the member of the audience, is necessarily an objective observer, separate from the action and from the characters. He views the story from the outside. In all but two or three cases, though, the Holmes stories are told as personal reminiscences by Dr Watson, and are inseparable from his subjective viewpoint; and to ignore this fact is to miss a great deal of the subtlety of the stories that has kept them so evergreen for so long.
Watson is constantly telling stories against himself. When he has Holmes say, "I have always done you an injustice, Watson. There are others," referring to a particularly obtuse witness, it is a sign of self-deprecating humour on the part of Watson himself, a humour which, in an objective retelling of the event, it is sheerly impossible to convey. Here lies the reason for the frequent portrayal of Watson as a blimpish buffoon, by Nigel Bruce and others; the fact that a blimpish buffoon could never have written the stories in the first place escapes the notice. The Watson we are supposed to see in the stories is Watson's own subtly exaggerated caricature of himself, made thus, of course, to play up the brilliance of Holmes. Try doing that on television.
I've just watched a longish Youtube video entitled "Sherlock is Garbage, and Here's Why," to which, with your kind indulgence, I shall not link. It's an hour and a half of pulling Steven Moffat and Mark Gatiss's adaptation to bits, and much of what it says is right on the money. Moffat's obvious contempt for his audience is nowhere else as clearly shown.
On one point, though, I would partly disagree. One of the charges that the video lays against Sherlock is that, rather than focussing on the method of the solution of each crime, Moffat and Gatiss make the show increasingly All About Sherlock. A similar thing happened with nuWho, where it became for a while All About The Doctor, but that started with RTD, so Moffat can't be held solely responsible for that.
But a cursory reading of the original stories shows clearly that, as far as Watson, the narrator, was concerned, it was indeed All About Holmes. He relays faithfully Holmes' methods, and details his solutions, but the purpose, for Watson, is purely to show how exceptionally brilliant, and incidentally how thoroughly decent a chap, is his friend. This is one of those things which, in the objectivising process of adaptation to drama, can get lost; in Sherlock, it has not got lost, but it has lost its rationale. John Watson, as played by Martin Freeman, would never have written such accounts of Sherlock's cases as Doyle's Watson puts down. Nobody would, for Sherlock, as played by Benedict Cumberbatch, is not at all a decent chap and has nowhere near Holmes' capacity to inspire loyalty and devotion. Add to this the fact that Moffat is such a lazy writer that, as the video witheringly points out, most of Sherlock's solutions to cases can only be explained by magic, and the dislocations inherent in the adaptation become only too clear.
I gave up on Sherlock after he quite definitely and unequivocally died in front of Watson in the season finale, only to be resurrected in the final two minutes, and the first episode of the following season, after pouring scorn both on various convoluted theories as to how he survived, and on the groups and individuals who put them forward (representing, as the video says, the actual fans of the show), revealed Moffat's actual answer; it doesn't matter how he survived, so don't bother thinking about it. I gather it didn't improve, so I won't be going back to it.
I shall, however, often return to the original stories, in which John Watson, that competent and intelligent medical practitioner, chronicles the exceptional yet easily fathomable achievements of his friend, and pokes gentle fun at his own rather more pedestrian thought processes; and I shall wonder if, between the buffoonish portrayals of Bruce and others, and the capable yet somewhat solemn renditions of Burke and Hardwicke, there will ever be found a way of adapting the stories for drama which can combine the two aspects in the right proportions.