On recumbent trikes, vs road bikes which is safer down hill?
Safety on descents is more a matter of your state of mind than any objectively measurable phenomenon.

I will say that I have descended pretty damn fast on a road bike, and faster on a tadpole trike. This had less to do with being on three wheels and more to do with having an extremely low center of gravity on the trike. I've also never experienced speed wobbles on my trike--I'm not even sure that's possible.

You do not tip on a trike or bike when you're going in a straight line. If you're going through curves, the fact that you cannot lean into a turn on a trike means that you can flip the trike instead, and at higher speeds, you'll flip the trike on shallower turns.

Typically, tadpoles have two front brakes, and for the most part, this is a benefit. Braking at very high speed will steer the trike into the braked wheel; if you apply both brakes, then the slightest discrepancy in the application of the brakes can cause a dangerous back-and-forth when you're whipping downhill.

From what I've seen, a tadpole can be designed with a lower center of gravity than a delta, which improves stability.

If the road surface is too broken up or littered with debris for me to descend at full speed, then I don't, regardless of what I'm riding. I suppose in the abstract I would feel more endangered hitting a given rock at speed on my road bike than I would on my trike (I also have much fatter tires on my trike). It would also be harder to avoid the rock on my trike (three tracks vs one). But that's a hypothetical I am not eager to test.