For excel: our instance doesn't deal with a lot of excel review, so I can't personally speak to whether it's any good, but just want to point out that unless the print area and print breaks are set well on the excel file (including how much can fit on a page), it often doesn't convert to a Proof well.
For word and other word-like files like powerpoint: we do get users who aren't really thinking about what they want in this regard. Workfront Proof is excellent for review and marking up only. We do get teams who prefer to their reviewers to also make the changes at the same time as they review so that changes can reflect immediately on the file -- I wonder if this is the feedback your users are also giving you? This really isn't how Proof works, or offers to work, so it's important to be able to clarify this when implementing teams (whether they want to change as they go or whether they want a commentary only).