Aren't they just as stable as regular rockets, but it's very inconvenient to put the thrusters on top? It's called pendulum rocket fallacy and from what I can tell that's the case.

At some point (some) people though rockets with the thrusters near the top would be more stable. See e.g. this rocket by Robert H. Goddard: https://en.wikipedia.org/wiki/Robert_H._Goddard#/media/File:...

But it turns out that it doesn't matter whether you place the thrusters high or low, stability-wise. What matters is that the center of mass is in front of the center of the center of pressure.