TrueTime is a very special case. I have not seen anyone else who has the same hardware infrastructure required to provide guarantees related to wall-clock time. Spanner also trades off transaction latency to create the wall clock time guarantees, so its not magically free.
It isn't, I haven't said it is. The point is, there are disadvantages you are writing about, that aren't really disadvantages of the protocol itself, but disadvantages of the common implementations.
