I've always found SMF to be faster and to use less resources than other forums, and I have more trust for it. I only ever had one major attack on it, and that was early in SMF's product life.
One of the things about SMF I like is that the templates are just PHP files. That's how I think, that's how I work, and it's one of the things that pisses me off -- when people for no good reason slop some derpy "template system" atop a language that was created TO BE A HUFFING TEMPLATE SYSTEM! Smarty, handlebars, whatever -- incompetent trash that's just harder to work with.
If functions, variables, strings, and scope is "too hard" for people writing templates to slice up their markup with, they probably shouldn't be making templates!
The big paid forum software all tend to be utter garbage that I wonder how they have the brass to charge money for. vBull being the poster child for this. Hard to skin, painfully slow on the back-end, lacking in features out-of-box I expect out of box, etc, etc.
Not that other alternatives -- xenforo for example -- are any better. There's a reason places like DigitalPoint have to throw extra caching like Varnish at their sites resulting in an often broken experience from updates not overriding cache properly.
There's also the matter of trust, and to put it bluntly I have never trusted phpBB. That the old versions didn't suport even the most basic of things out of box (signatures, avatars, attachments) sending people diving for third party mods, said mods neutering the upgrade path resulting in old/outdated copies remaining in circulation far past their freshness date, which is how that pesky "nevernosanity" (aka Santy) worm took down half the blasted Internet, infecting websites that weren't even running phpBB if they were even on the same host. Blasting past any alleged security between domains of shared hosting.
Sure, that was 15 years ago and most of the things that made people screw over their ability to upgrade have been fixed for over a decade, but I still just don't trust phpBB.
That and I'd have a LOT more work than I already have to drag phpBB kicking and screaming into accessibility compliance given that they derp everything together in PX. With SMF I'm comfortable deploying the vanilla skin as a stopgap when I need "the forum up yesterday". I would have to do a new complete skin from scratch before I'd even consider putting a phpBB forum into production use.
I mean sure SMF has its own problems, but weighing all the different issues I've dealt with in forum software over the years, and observing problems other forums have, SMF remains my go-to as it has the least number of problems in the areas I care about.
Though the permission system is so powerful it's easy to get lost. It's not over-complicated, it just matches task complexity without dumbing things down any.