Generally speaking, Autodesk Fabrication is probably the most reverse compatible piping program I've ever seen. However, this is a tricky question and that statement is only 100% valid while having access to the database that originally created the files. If you are saying that the 2016 users and 2015 users have entirely different databases, then yes you can expect all sorts of erroneous problems.

In the "edit configuration" dialog, there is a setting for specifying the programs behavior while opening a drawing. This is typically set to "No" or "Prompt". In the case of NO the drawing is read using the database the person currently opening the drawing is configured to use. This scenario will likely create connector and material issues likely changing the overall takeoff/OD of objects. 

If it is set to YES then the objects already drawn should theoretically maintain their attributes as they were originally drawn at the time. However, modification of those objects will likely break the link to the DWG nested original database info and make them start trying to utilize the users current configuration; causing takeoff/OD errors.

In short, CADmep objects aren't really dependent on the DWG version persay and will make every attempt it can to give you something. You can still migrate/configure Autodesk Fabrication to go use a very old database and open files you worked on 4 years ago, but I wouldn't expect to open those files up (error free) using a newer database that didn't maintain the information that was used in the past.

FYI, we have been working with a mix of a 2015/2016 CADmep & AutoCAD and logging in as FULL on both versions to the same database with no obvious adverse effects.