I wonder what you mean by that. Microsoft Word is even worse for producing complex documents now than it was in the heyday of FrameMaker. Its Word users' inability to enforce the separation of presentation style and content means almost every Word document in existence has stylistic inconsistencies that are very hard to clean up.
There, I've fixed that for you.
Word does have bugs, and I hate it for that, but if you know how to use it it can do just what you want. The inability to "force" the separation of presentation style and content is what happens when you are trying to cater to a mass market of users who don't know what they are doing. But if you do know what you are doing, Word is completely able to
support the separation of style and content.
It's just like safety features on power tools. Those safeguards are there to help prevent the clueless from injuring themselves. But if you are an expert you can discard the safety features and have more control over what you are doing.
For what it's worth, I can make very complex technical documents in Word and see no reason to look anywhere else.