Windows and doors from a single source or not? – Experiences?

I recommend choosing the stress elements (front door, tilt-and-slide elements) made of aluminum, the rest can also be PVC (I myself would choose aluminum throughout, but the average Joe has too much money and prefers to buy cheap and more often).

May I ask what is bad about modern PVC windows?

According to Google, the service life is also supposed to be around 25-40 years.

Basically nothing. But didn’t claim that either.
You can also buy a lift-and-slide door or front door in plastic. But for these areas, there are better suitable frame materials – namely aluminum.

Better leave out the 25. 40 years is no problem at all.

May I ask what is bad about modern PVC windows? According to Google, the lifespan is supposed to be around 25-40 years. I might only worry about the foil on colored frames.

As a former window dealer (all materials) and manufacturer (only aluminum), I don't need to ask Google about this. PVC is, as its common name already says, a plastic. The durability in terms of decomposition time is more of an ecological than an economic argument against PVC. Technically, plastic windows also don't need to be replaced much earlier, but visually they do: PVC windows look so fresh in the first three years that even I, as a professional, can only see the difference to aluminum on the surface with the right glasses. But after about 15 to 20 years, PVC windows look as if the frames were scrubbed with scouring milk during window cleaning due to embrittlement caused by the evaporated plasticizers. Aluminum windows, on the other hand, still look factory new after 50 years. For colored frames, you don't need to use foil on PVC; coextrusion is clearly the better way. And aluminum can also be foiled—wood look is only convincingly achievable with powder coating at a disproportionately high cost.

My wife wants gray windows (inside and outside) and in the end the surcharge for plastic+foiling compared to aluminum might not be that high after all...

I like coextrusion, but it’s probably not done that often?