What is the worst part about working in the facilities engineering sector?

Keeping yourself and your staff motivated is the toughest part of working in this sector.
Facilities is an ongoing, never ending job.
If you are working in Projects, when your project is done, it's done.
You are able to move on to the next project and your successes are clearly measurable ie - finished the project on time/under budget etc. 
But you are only successful in the facilities sector when everything is running as it should, and a client/boss will not give you much credit for that as it is the minumum expectation.

The toughest part of working in the facilities sector is trying to motivate upgrade and maintenance expenditure.
To do the job correctly ongoing investment and upgrades are needed, however the "bean" counters do not see it the same way, they tend to see all expenses in this regard as unnecessary expenditure.

Facilities engineering is really an operation $maintenance.And you must know maintenance is a thankless job and you try to make it a thank you job.