> Very much depends on their proof of stake implementation

Exactly, if someone with >50% power can essentially double spend it makes the currency as a whole almost worthless. As a result it's very much in the creator's interest to either not have >50% or to come up with a distributed consensus algorithm where a majority party can't double spend (very hard).

I agree. It's more like they're retaining control of the money supply like a central bank does. It give you the power to indirectly influence the price / exchange rate of the currency, but you're not involved with every transaction.