Does this mean the underground sections can be as long as you want or do we still have to go aboveground every x tiles? Is there a downside to staying underground as much as possible?
How does this work on Aquilo, do pipes underground still consume heat/get frozen? Do the transition pipes still consume 150x as much as regular pipes?

There is no downside to staying underground with your pipes. None whatsoever, but they do have the same limitation of not being able to be placed in empty space or on lava (unless another mod changes that)

I have not tested if they are heated or frozen, I'll do that eventually. I think I'll make them not require heat, like they are buried far enough underground to be sufficiently warmed.

Just confirmed, the pipes don't freeze on Aquilo. I will also update the mod so that the pipe-to-grounds have the same heat consumption as normal pipes.