I can’t speak to where Jonathan got all those extra entries under Meta-rig. In fact, they may even have been a standard part of an earlier incarnation of Rigify. On the other hand, they do have a rig expert or two at Blender Cookie, so who knows?
But I can tell you that FlexiRig was built from a foundation of Rigify. If you look at the UI script, it’s the same but with extra functionality added on for resizing various things.
One disadvantage to FlexiRig is its lack of IK/FK matching/snapping. Rigify does have this, so from a pure flexibility POV, Rigify is better if you have to switch back and forth between IK and FK in the middle of a shot.
Yes, there is the disadvantage of having to build your own face rig. But I think the most time-consuming part of face rigging isn’t building the rig, it’s weighting it. If you stripped the mesh off FlexiRig and added your own, you’d end up weight-painting it anyway. Using Rigify as a foundation and rolling your own face rig means you’d be laying out bones and working out controllers. How long that will take depends on how much experience you have rigging and how well you understand the process of planning a rig. It took me about two months to work it all out, but I cheated like mad in the end. I built one complete rig for body and face, then used it on a whole cast of characters for the Oh Balls! concept I’m working on. I don’t guess that could be done for humanoids.
It would be nice to see a rigging system that does for faces what Rigify does for bodies and maybe someday there will be one. I feel your pain.