According to Bostrom, brain-computer interfaces are unlikely to yield superintelligence.

This strikes me as slightly surprising. From a technological standpoint, I would have thought that a hybrid of biological and machine intelligence would be likely to have the best aspects of each, by which I mean the best aspects at the time at which the BCI is created, rather than trying to posit that biological intelligence has any fundamental advantage that sufficiently advanced computers can never overcome. A fairly close analogy is how teams of a competent chessplayer and a laptop chess program can beat both the best humans and computers with far more processing power.

Admittedly I don't know much about BCI technology, but I have heard promising things about optogenetics. Having to undergo brain surgery is a problem, but the extent of this problem seems to depend upon to what extent the interface needs to penetrate into the brain rather than just overlying the surface. If bootstrapping to greater levels of intelligence required repeated surgery to install better BCIs then this might be problematic, but intelligence gains could also be realised by working on the software, or adding more hardware, or adding more people to a swarm intelligence.

Of course, in the end it would transition to a fully or mostly machine intelligence, either through offloading increasingly more cognition to the machine components until the organic brains were only a tiny fraction of the mind, or though using the increased intelligence to develop FAI/WBE. But that doesn't make BCI a dead end, so much as a transnational stage.

Finally, in the last few years, Moore's law has started to show signs of slowing, and this should cause one to update in favour of BCI coming first, as it is probably the path least dependent upon raw computing power (unless de novo AI turns out to be far more computationally efficient than the brain).

As far as social constraints go, I don't think it would be all that hard to find volunteers, and in fact there is a natural progression from treatment of blindness, mental illnesses and so forth through to transhumanism. Legal challenges are perhaps a more likely problem, but as previously mentioned, medical use will likely provide the precedent to grandfather it in.

Note I'm not saying that this is necessary a desirable path - FAI is preferable - I'm arguing it seems at least somewhat plausible to come first. Having said that, in the event that progress on FAI is slower and other existential threats loom, than BCI could perhaps be a sensible backup plan.