I tune BMW MS41, MS42, and MS43, N/A and added FI... and yes, they do adjust... but they're actually adjusting for mistakes, as well as minor environment conditions.
Add too much fuel you get knock, take away too much, you get knock, too much ignition advance, you get knock.
Yes, knock sensors pick it up and pull ignition advance... which just burns more fuel down the exhaust, and raises EGT and not power. Sensors help keep it safe, not powerful.
Still have a lot of tuning potential left in variable valve timing, electronic throttle improvements, etc.
Now of course, stand-alone systems or even some piggybacks have made it much easier, almost silver spoon tuning.
Good diesel tuning is obviously tougher, but most TDIs can easily stand a simple +10% or +15% multiplier to fuel maps, smoke limiter, and torque limiter maps, and not even smoke, or be dangerous... without even touching turbo maps or any other maps.
Of course that's not a 'good' tune, but it'll work.
I do agree with you, they tend to be the same, when you try to get in depth. And neither can you just "OK let's learn to tune, give me info" and tune in a week. It'll take months to years.