That's not been my experience. As long as you don't do something silly like trying to mix brands, index shifting isn't hard to maintain. I volunteer at a local coop as a bike mechanic every Saturday and see dozens of different systems per day. If the shifter still clicks or can be made to click, the indexing can be made to work like it did from the factory. Even Grip Shifters...which are universally hated by every mechanic I've ever talked to...can be coaxed into working properly. They can usually even be coaxed into working well.

About the only system that can be made to work well are Suntour shifters but those didn't do a good job from the get-go. That's the reason Suntour went out of business.