The FHI paper, Eternity in Six Hours, is very optimistic about what can be done:

In this paper, we extend the Fermi paradox to not only life in this galaxy, but to other galaxies as well. We do this by demonstrating that traveling between galaxies – indeed even launching a colonisation project for the entire reachable universe – is a relatively simple task for a star-spanning civilization, requiring modest amounts of energy and resources. We start by demonstrating that humanity itself could likely accomplish such a colonisation project in the foreseeable future, should we want to, and then demonstrate that there are millions of galaxies that could have reached us by now, using similar methods.

Is this paper reasonable? Which parts of its assertions are most likely to be mistaken?

This question was inspired by a conversation with Nick Beckstead.