I started off avoiding mod.rs because it’s the old way and I prefer having the module name as the filename. However, if the module needs a folder for submodules anyway, then there’s a reason to tuck it away as mod.rs, especially if not doing so leaves lots of duplicate names (a.rs, b.rs, c.rs, …, a/, b/, c/).
But then I don’t really like to have much else in mod.rs other than mod declarations and pub use. Maybe a utility fn or a not-unwieldly implementation of struct “Foo”, the module’s namesake.
I avoid using mod.rs because then I’ll have a bazillion mod.rs files. It’s what I hate about python modules. I would rather have a thingy.rs and then a thingy folder than many files named the same thing distinct only by their path.