If it assumes remaining work is equivalent in time to done work (notion of velocity), it's (in my view) very optimistic. People learn the nature of the beast as time progresses. Collective learning happens slowly. A lot of important work is usually only discovered and planned in the second half of the actual (not original) timeline. A lot of important stakeholders are naturally only introduced into projects close to its finalization, which creates a flurry of new activity and discoveries. This makes projects late.
But where that real halftime is - most people I worked with rarely know. I was no better, I also rarely knew.
The only way to manage this I found was the basic rule: build, demo, decide what to do next. Don't get attached to original backlog and grand plans for the future. It won't work that way. Just build and make it work, launch ASAP, get decisions on a ~weekly basis done. Ignore long term planning. It's not helpful.
Sadly, few business folks want that of course :)