I have both Di2 and mechanical bikes.

Di2
My carbon Di2 is great on fast group rides and on rolling, hilly rides. It's like clicking a mouse to shift, and I can do it with my ring fingers from the hoods. The rear shifts great under load on hills, and the front shift is quite quick and drama free. I'll sometimes shift the front for just a 30 foot high climb. I'll often shift the rear for just 2 or 3 pedal revolutions, then shift again.

To keep up with fast rides, I'm always hunting for my best cadence, so I shift a lot. And when the other riders are standing and grinding up a short hill in a too-high gear, I've already dropped to my 34 chainring and a big cog. And way back in the draft, I'll go 50-11 or 50-12 and pedal very easy at a low cadence to both save my legs, and avoid coasting. (It's easier on the following riders when the rider in front doesn't pedal & coast repeatedly.)

Mechanical
I have a gravel / all-day bike with a triple crank. It's great for really low gears on steep rides. And the 39 middle chainring is really nice with a 12-25 cassette for all-day flatter rides--I have very close shifts that just change the cadence by 5 or 6 rpm. (If Di2 had a triple, I probably would have done that.)

I'm riding at a more relaxed pace, and don't need to shift quite as much, and can afford to wait longer to do a shift. So the mechanical shifting is fine for these rides. It's interesting how quickly my fingers adapt to the very different shifting when I switch bikes.