The biggest downside to staff IME is that you are almost constantly reminded that you are a cost and at danger of being cut.
OTOH, the biggest upside is the domain knowledge, and in particular, how companies (ie customers of the line engineers) really deploy and run their systems, which IME is often very different from how the line engineers think they run them.
Line engineers tend to gloss over the myriad of external influences that affect how easy their systems are to deploy and integrate (eg how many FTEs doe it take to operate your systems, how long is an acceptable maintenance window for applying fixes, how long does it take to roll back if a fix fails, etc).
An experienced staff engineer introduced to a line team can be a huge asset, and will ask the right kind of questions, ie those that the customer is often most interested in.