Moreover it is virtually impossible I would think to get a Visitor visa these days and then do AOS like this thread talks about.
You need to break this into two parts.
1) Getting Tourist Visa.
2) Marrying someone here on a Tourist Visa and then doing AOS.
Number 1. Ease of getting the tourist visa depends on where the person is from. All persons from FSU have a somewhat more difficult time getting tourist visa than do persons from many other countries.
And, the person will be denied the tourist visa if there is any hint that they may have a romantic friend in the USA, and that they are getting the tourist visa and later plan to file AOS to circumvent the longer wait apart from going the K-1 route.
They will also have trouble getting the tourist visa if they are married to a USA citizen because it is suspected they are probably trying to circumvent the longer wait apart from going the CR route. However, the Embassy staff may take pity on the couple and grant the visa, since they know eventually the person will make it to the USA after the CR process.
Number 2. This is the easy part. There are no restrictions against marrying someone here on a tourist visa. And the rules specifically allow for AOS for those who arrive on a tourist visa and marry a USA citizen.
Further, the USCIS has completely stopped trying to determine intent. i.e. they no longer try to determine if the couple had the plan to arrive on a tourist visa and then AOS. The only pitfall is if the married couple would be stupid enough to hint at or voluntarily provide this info in the written applications or the in person interviews.
So number two is not really a problem at all.
The problem is in number 1.
Note: Even as number two is not a problem . . . it is really not a fair procedure compared to those who have to go through a lengthy period apart when going the K-1 or CR route.
There are three categories who can AOS without going the K-1 or CR routes.
1) Tourist visa
2) Work visa
3) Student visa
The tourist visa is the most subject to abuse. Because many in that category did meet abroad, plan to marry, and go the tourist route.
The work and student visas are less subject to abuse because in most cases the person comes to USA without knowing anyone here, at least in a romantic sense.
Then after working here or studying here, they just follow the standard procedure of dating, find a compatible partner, and get married.
There is probably a case for terminating the ability to AOS from a Tourist Visa because the tourist really hasn't established a connection with this country. They probably should go back home at end of visa and start the K-1 or CR process.
However, there is little logic in forcing a student to stop their education or forcing a worker to stop their work and go back home after finding a mate in the USA.
Full disclosure: I did marry a person who was here initially on a Student Visa and then was working under the Optional Practical Training program at the time of our marriage.