MSC cruises are very inexpensive.

Carnival gets some bad reviews; but their burger place is really good. Their buffet has something likely a Mongolian grill. I don't love Carnival's main room dining; but their buffet places are pretty good. I think the criticisms of Carnival are a little overstated. Their old ships aren't great; but neither are Royal's/NCL's, etc.

Regarding MSC. I have not cruised on MSC. Historically they have been a European only cruise line. They are trying to break into the US market. That is one of the reasons why you can get really good prices on them. However, I have not heard good things about them. I do cruises in the Mediterranean every year with my company. MSC and Costa are, by far, the cheapest. But I won't use them with my company. I will book with Royal Caribbean, Celebrity or NCL only when I'm in the Mediterranean. If you do a search for reviews of the MSC ship, you will find the complaints...and the reasons I won't use them. They are geared towards european travelers. As such, the food is quite a bit different than the ships that target a US audience. I have had quite a few guests travel on them with not good reviews at all about the food on them. Often there is more smoking (at least when in Europe). There are not as many amenities and things to do on them as Royal, NCL or even Carnival.

Honestly, if I were going for a less expensive cruise, I would recommend Carnival over MSC. But that's just my opinion.