Spoiler non-linear aerodynamics
I learned early on in my time as a flight controls engineer that spoilers do just that, spoil lift. They are not very effective as direct drag inducing devices. The way that they work when employed for speedbraking is different in-air vs. on ground (both resulting from their lift dump characteristic):
In-air, deploying spoilers dumps lift requiring increase pitch attitude (AoA) to continue to fly at the same flight path angle. The increased AoA is what really brings on the increased drag and slows the airplane. If spoilers are deployed without increasing pitch attitude, the lift loss will result in lowering the flight path angle. On a related noted, care must be taken as to how quickly spoilers are deployed when in air as the lift loss can be a real surprise to the passengers if done too fast! A feature on some newer models that include fly-by-wire control augmentation is to slow in-air spoiler deployment so that a quick extension of the level on the flight deck does not result in the bottom dropping out for everyone in back.
On-ground, deploying spoilers dumps lift allowing more effective wheel braking. The spoilers by themselves (without wheel brakes) don't contribute much to deceleration during rollout.
An interesting thing about spoilers is that at certain angles of attack extending them slightly will actually increase lift. I suspect that was what was happening during the 747 flight testing mentioned above. This is because they act like vortex generators energizing the flow over the wing and helping keep it attached. Once spoilers are extended beyond a couple of degrees their net effect is a loss of lift.
On the subject of pitching moment from spoilers, one key aspect for a swept wing airplane is the coordination of inboard and outboard spoiler extension. Get the mix right and the net pitching moment is fairly neutral. Too much inboard spoiler compared to outboard and the airplane with pitch nose down. Too much outboard compared to inboard and the reverse will happen - i.e., a nose up tendency. Airplanes with fly-by-wire augmentation have the added degree of freedom of combining elevator motion along with spoiler deployment to tailor the pitch response.