Hopefully the experience of others will help you, but I can answer from a McAfee perspective...
When 3rd party apps exhibit symptoms due to the presence of our software, it is more likely to be a coding problem within the 3rd party that's being exposed by our "newer" code.
There are various scenarios where installing our software might exhibit problems in another's software - let me cite a couple common examples:
- they use a legacy filter driver and relied on load order, but the OS provides them no such guarantee for load order
- they use an API having credentials provided which gets processed by the kernel in a manner where credentials are not passed on, and we enforce an Access check to avoid propagating a vulnerability
There is little point for us to research those symptoms because we can only look at what our software is doing, and of course we'll see that it's not doing anything abnormal. The "Why" behind the 3rd party failing can only be deciphered by the vendor. And if they tell you "It's because McAfee is installed" you should slap them and say "look harder!" because they need to provide a technical analysis, not a circumstantial one.