So, $YFI is a governance token with a market cap of $922M atm, almost 1 billion dollars. So how does this governance work?
There's nothing on the main web site: https://yearn.finance/ 
Nothing in docs.
Through some search, I found https://ygov.finance/vote 
but again, no

explanation about what it is and how exactly it works.
So it's not clear what exactly investors are investing in.

Solidity code for yGov is provided here: https://github.com/iearn-finance/audit/tree/master/contracts/yGov

Code which actually deals with proposals and voting:
https://github.com/iearn-finance/audit/blob/82694ed21b83606e8ce5ae0146f48fa6d372d25d/contracts/yGov/YearnGovernanceYFI.sol#L653

does nothing to execute a proposal. It just records number of votes.

OK, let's start from other direction. The $YFI token contract:
https://etherscan.io/address/0x0bc529c00C6401aEF6D220BE8C6Ea1667F6Ad93e

mentions governance, so how is this governance implemented? You can find that the current governance contract for $YFI is

https://etherscan.io/address/0x026d4b8d693f6c446782c2c61ee357ec561dfb61
which is TimelockGovernance, essentially a governance-updater contract. The governance for TimelockGovernance is set to https://etherscan.io/address/0xfeb4acf3df3cdea7399794d0869ef76a6efaff52#readProxyContract
which is GnosisSafe 6-of-9 multisig among normal addresses (i.e. physical people).

So, as far as I can tell, $YFI doesn't actually govern anything, it just allows you to cast coinvote-style votes on a forum of sorts. And then 9 guys who have keys might execute a proposal, if they like. They aren't legally required to, and it's not even clear who they are.

So, basically, people are willing to pay $900M to access a forum.
This is several steps back from 2016 The DAO, which proposals which can actually be executed on chain.

I wonder if people buying $YFI understand that they have no formal power of any sort. It's all at discretion of Cronje and his cronies.