Biggest issue:

Kill Team is written by a completely different team.than true 40k, and those team members do not interact within the rules space (the Specialist Games division are literally treated as different members of the team) and the nature of things like Warcry, Kill Team, Adeptus Titanticus literally means that there only NEEDS to be the single major core rulebook, which the team writes once and is done.

The very nature of 40k codices is they write several codices at once, in waves, each codex written by a different author in that "wave", and there is no "rules bible" that enforces writing rules identically, etc.

Until they write codices collaboratively, we will always see waves of codex creep