No visa EVER guarantees entry. So yes, you can be denied entry the first time, the 2nd time, or the 10th time. It's at the discretion of the immigration office you are standing in front of. The only thing a visa does is determine how many days you will be stamped in for if you are admitted entry. METV is safer than trying to enter for ~9 months visa exempt, but it's still not a guarantee. Many, many reports of people being stopped and interrogated on their 2nd or 3rd METV entry after doing a 60+30, then a short trip out and repeating.