Lets take Path of Exile as an example of what legendary/unique items should be like. IMO the game has the best uniques compared to every other ARPG, they have proven that you can have different build-enabling items without designating a whole set around them. It is excessive, a relic from the past. Lets be honest here, sets were not that big of a deal even in Diablo 2. So why its heirs are so desperate to include them?
Diablo 3 has to be the example of sets done the worst where they are so incredibly strong they are the end-all of everything, while at the same time heavily restricting your build and every other item which is not a set one.
Grim Dawn is not that much better honestly. At least they have rare(green) items which could be on par so they are not the end-all but the system is still flawed because it requires you to hunt for hundreds of set pieces in a game which was not built to support trading. Also the sets there are quite bland but this is a different topic.
This poses a logical question: why do we need sets when PoE proved you could enable different builds and ways to play the game without them? Why is restricting your choice by forcing you to wear matching set pieces a better solution?