I've been running MS Office since Office 96/97 and can't remember having any problems with versions since, except for ver. 2007 under Wndows 10. Is one reason why I upgraded to a later version(s) as I noticed some strange behavior from Outllook 2007 under Windows 10.

After the final upgrade to version 2016 I've not had any problems. One earlier version I did have problems with the validation and was required to completely remove this installation as well as the 2007 installation in order to install ver. 2016. It was suggested by several people to completely remove MS Office from my PC before the final installation of ver. 2016.

So I think perhaps most people don't have any problems with MS Office and even running multiple installations. However there are a few people who do, which is not a common occurrence.

When I contacted Microsoft support in order to question the legitimacy of the MS Office purchased from an eBay seller I was told there were many illegitimate copies being sold on eBay.

To check the Office 2016 version installed on my PC they had me download and run a remote program that allows them to take over my PC remotely (after giving them permission). They then perform various tests of the Office installation checking files, etc. MS also checks the validation key code which I provided.

Everything tested ok as one hundred percent legit.

Some people do have problems with MS Office installations. If you sift through some of the forums and the Microsoft website you will find people who have had problems with their installation for one reason or another.

It's often suggested when having problems related to ms office is to completely uninstall. There is a utility program that can be downloaded from microsoft.com that will uninstall all versions of MS Office installed on your PC. There are also instructions on how to manually uninstall MS Office from your PC, completely deleting all traces of it's installation(s).

It's sorta like something doesn't seem like a problem until it actually happens to you, then it becomes a problem.

E.g. my car runs and runs just fine for years, as I maintain it, everything is ok. Until something breaks and becomes problem.