It also means that said government has unfettered access to your books, reports and IP. They know everything you do. This can be significant in companies like Facebook, that are critical to propaganda efforts and are increasingly gamed in systematic ways by state actors. If they know everything about FB internal operations, they can probably exploit its weaknesses in very effective ways.

EDIT: to clarify, I used FB here as a generic example, I am not alleging that Milner got that sort of access at Facebook specifically - although it is possible, as others point out downthread. Corporate espionage certainly gets much easier when you have a seat on the board and/or you can do due diligence.