Some people that use Office really push it hard, they've got macros, indexes, all sorts of junk that's edge-case. The converters do not always pick up on this stuff, or if they do they subtly mangle the formatting enough it's all wrong.

For example, if you have an index then a tiny kerning shift can bump an item to a new page and screw up your numbering.

It really depends on the types of documents you're dealing with. Some are hell, others are a no-brainer.