Unsurprisingly, each camp claims the advantage. Like you, I believe there is little difference. This was never a factor in my decision of which to use.
I would rephrase this as “Installing third-party software isn’t a problem, unless someone has nefarious intentions.” That goes for any app you might get from the App Store as well.
I’d say this is probably true, because the MacPorts philosophy is to be independent from macOS as much as possible. Going forward, I think this gap will narrow.
There are pluses and minuses to both systems, and making a final decision isn’t easy. To be honest, one reason I chose MacPorts is that I couldn’t get past the cutesy, unintuitive terminology of Homebrew (cellar, bottles, taps, etc). I dimly remember getting into some trouble with versions of some programs delivered with OSX that didn’t work well with Homebrew, which is, at least, less likely with MacPorts. Like OP, I also disliked the ruthless automatic updating of Homebrew, and I wasn’t smart enough to find the fix posted by @cornchip.