To clarify, I'm just taking exception to when people say that the 3 part update isn't "reliable" and it b oils down to what the semantics of what someone considers the word "unreliable" to actually mean.
To me, "unreliable" means that it should never be trusted to work. As I said throughout the article on running totals, one of the rules is that you first have to make it all work. Once you get it working, then it will work reliably.
It's like taking into consideration the problems that people have documented about the relatively new MERGE statement. It sometimes does strange things that cause the code to not work as expected. Does that make it "unreliable"? Perhaps. I like to think of it as it being very reliable in giving you problems if you use it a certain way and very reliable in giving you correct answers if you use it in another.
To quote the article...
It's just one option that happens to work well for running totals and running counts.
Interpret that as meaning that the 3 part update is reliable for "normal" Quirky Update running totals and counts. If you do something strange with it, then it will be "unreliable".
--Jeff Moden
Change is inevitable... Change for the better is not.