Hmm, difficult one! I think the risk here is getting caught up in definitions rather than focusing on what a MCF is actually for. My understanding, as someone who has worked both in maintenance and flight test of aircraft, is that the MCF is primarily there to test/prove systems that cannot be tested adequately on the ground. A similar approach is taken to test flying ; it is generally not undertaken unless the evidence needed cannot be obtained any other way or certification demands it.
Even with complex FBW aircraft, most if not all primary and secondary control surface movements can be checked thoroughly in terms of correct deflections for different flight conditions using test modes and a pitot-static test set to simulate speed and altitude. Furthermore, you would hope that maintenance work requiring a MCF would not result in a significantly higher than usual risk of the control surfaces failing to work as they should! If that were not the case, then the efficacy of the post-maintenance functional checks should be called in to question.
The only things that are not easy to test on the ground are functions that require or respond to movement (e.g. roll, pitch, yaw or longitudinal, lateral or normal accelerations), or to navigation sensor inputs. For these you have to be in flight because there is no easy way to exercise the relevant sensors and associated control loops on the ground. This would include systems such as the autopilot, auto-throttle, auto-braking and yaw damper, and also navigation functions such as FMS navigation route steering, VOR radial or ILS localiser capture, etc.
In answer to the two systems you mentioned:
Spoilers - if you have sufficient roll control and runway braking in the event of them failing, then they probably do not need a MCF. Also, there are usually several layers of redundancy in the spoiler system. An MCF would probably be required only if all the spoilers had undergone maintenance work in one go.
Tail trim actuator - a key part of the autopilot on most aircraft, and on some aircraft it becomes more critical if reduced longitudinal stability is employed in the cruise by pumping fuel aft (e.g. A340). Also, if you get a pitch-trim runaway it can be a nightmare. On the other hand, the basic trim functionality can be checked easily on the ground, with the exception of the autopilot modes that use it. On these grounds I would opt for a MCF, but focus on the operation of the autopilot vertical modes.
Hope this helps.
WF