One problem with grassy lines is that when you need to temporarily use replacement buses (like in cases of issues with the electricity or maintenance, etc.) they can't drive on the lines and have to take the regular roads with the cars.
At the moment one tram line in my city has been interrupted for a few weeks and that's how I noticed that the grassy line was making things more complicated for the buses that are used instead.
I think that's one reason why most cities only put a few sections with grass.