For me the issue with "the most". Unfriendly AI is a worrisome existential risk, but it still relies on technological breakthrough that we don't clearly estimate. While "bioengineered pandemic" is something that in the short-term future may very well be possible.

That doesn't mean SIAI isn't doing an important job - Friendly AI is a hard task. If you start to try to solve a hard problem when you're about to die if you don't, well, it's too late. So it's great SIAI people are here to hack away the edges on the problem now.