The Scylla and Charybdis of writing Canon-based Sherlock Holmes stories, if I was going to venture a guess, would have to be how you portray Sherlock Holmes and how you portray the Victorian era. Because even if you have a really good feel for our hero, the times in which he lived often seem an unfathomable mess, just as any culture in any time period, for one reason and one reason alone: One size does not fit all.
In earlier days of my Sherlockian life, I remember writers finding some etiquette book and claiming the rules therein applied to everyone in London. Growing up in a time when Amy Vanderbilt was still the acknowledged American authority on proper etiquette, with books on the subject in every bookstore, I knew that to be complete BS. Why should our working-class Victorian counterparts follow book rules when nobody I knew did?
Sherlock Holmes's clients were definitely not all manor-house types. And even the murder mysteries did not all happen to the upper classes, as the trope-iest of murder mysteries since his time do. And even among the manor house class, the rules for any given house depended entirely upon the personality of the person who ran the house. If the person who ran the house threw his false teeth at his wife at the end of every meal, as is mentioned in the Canon, you know that the rest of the rules were probably loosely applied as well.
Human beings fudge their way around actual laws as much as possible. The thought that we're going to all agreeably go along with proper etiquette dictums is absurd. And that's an idea that needs to be applied to most things we know about Victorian life when it comes to populating a Sherlock Holmes story, now that we are so far divorced from that era. Take anything you read, think about the way an actual human would react to that idea, and go from there.
Think about all the articles that have been written by Sherlockians on "My Fifteen Rules for Writing a Pastiche," then think of someone in the future, who never read a pastiche of a Holmes story, finding one of those articles and going "Sherlockians of that era followed this strict set of guidelines for creating their fiction about Holmes." Usually those articles are one person's personal reaction to existing stories they've read and not a reflection of reality at all -- the reality was usually all those things we are told not to do.
In reading the works of Victorian writers, I am consistently amazed at how they were thinking the same things we were thinking, having the same problems with their fellow humans that we are having, and how certain things have never changed, despite the drastic differences in tech, hygiene, and entertainments. Those very human things -- love, anger, isolation, logic, mob-think, etc. -- are carried with us through the ages, and why certain story structures do as well. We crave the same thing from books the Victorians craved, which is why Sherlock Holmes still exists.
That point alone connects our present selves to the folk of Victorian England and reminds us that they weren't all that different from who we are now, as much as we'd like to put on airs of being all "future and evolved." They got addicted as we get addicted. Their hearts broke as our hearts break. And they loved a guy who could walk into a room and make the mysteries of life plain and simple, just like we do now.
They just got to do it with gaslamps and Hansom cabs.