In a flexible work environment, you need a way to determine the work output of the people involved in the flexible work environment. If you can't "see" the person doing the work, how do you know they are getting the work done?
While managers will often say they support a flexible work environment, their actions in the performance review show they are often more about physical presence in the office. Even, ironically, when the office of their employee is thousands of miles away and they can't "see" their work.
SMART goals, done correctly, focus on the work that needs to be done. They show the results of the work done and the results are compared to the goal set by the manager and employee for the work.
If the department is truly focused on the results as compared with the goals, both the employee and the manager have common ground that trumps physical presence in an office as a criteria for good work.
Setting up a SMART goal right takes work. It often requires negotiation between the manager and the employee to get the right balance of what is done. Plus, as the needs of the business change, the goals need to change as well. Getting the weighting right for goals that change in the middle of a performance review period also takes work.
When you work in a flexible work environment (or, better, a Results Only Work Environment -- ROWE), both management and the employee need to step up their game to get the goals right. It's far easier to just let all that slide -- and the flexible work environment goes by the wayside.
How much has your flexible work time been about time instead of results?