I've always heard the argument that things are more expensive in the Euro. I know people in countries where they switched to the Euro and salaries remained the same while prices of goods went up. I'm not sure why this happens (or even if it's true) but it's a reason a lot of people are against losing the pound.
Adopting it often makes central bank policy more challenging. An outright "Euro makes goods more expensive" statement is an oversimplification though, don't take it at face value and always ask for objective inflation data relating to the particular country in question.
And that's the key thing. They would be changing masters, probably for the worst.
As a former Euro supporter and now Euro sceptic I would recommend them to keep their lead in their own hand.
Their best option is have their own currency.
It's my believe now that my country just give away a very important part of their sovereignty (maybe the most important) to entities that have not our best interest in mind. I would not be against the Euro if I though that the people in charge is on my side.
As an example (even if I'm not greek), we have seen the ECB just closing the Greece bank system when they are supposed to protect it.
For anyone interested, there are some discussion now about the legality of that movement: https://diem25.org/thegreekfiles/