There's not really any extra layers of weirdness unless you're digging down into the nasty .cf files (which you probably never ever need to do). The m4 is just a detail (so you end up commenting with "dnl").
The relative complexity of the files is about the same—my postfix server config is roughly the same number of lines as my sendmail server config. And each line is just a single conf thing. Sendmail isn't really more complicated at all. It's just ugly.
I've setup both multiple times, and have worked with Sendmail since 1994. Postfix config files are much simpler.
To configure sendmail, you have to do extra layers of weirdness, like deal with "m4". That's mental overhead you just don't have with Postfix.