If you reflect on the previous rules, after making an achievement-1 post and being verified by the greeter, then the user can be said to be a free user adventuring on the platform. So what are achievements-2, 3 and so on for?

This is what I saw the problem to be with the previous approach. When it was first launched, the Achievements represented learning and users were rewarded for their learning. Once users stopped being rewarded, they stopped learning and as you say, they became free. This put a lot of responsibility on the Greeters to assess somebody from a single post. I'm certainly repeating myself here but it's one of the main reasons for the change.

What are the disadvantages of users who do not follow the certification?

Different communities will have different requirements. So from a community point of view, most are still open to the "greeted" status although some want a little more confidence in who enters their community. So I would say that the disadvantage is that users without it will find these communities more difficult to join and will encounter additional "suspicion" when trying to do so.

The certification is aimed to give moderators additional confidence in the users that hold it.

It'll be interesting to see what happens with the accounts that previously used Achievement 1 as their "free ride" and how communities will respond. Will more communities perform their own additional "background checks"? Will more communities decide that there are too many suspicious users and move to a different Group?

I know that I'm explaining myself very badly. So perhaps an old comment of mine will explain better..

https://steemit.com/hive-172186/@the-gorilla/seicmi