I don't believe I've ever seen 4 junior engineers who don't know what they're doing build anything other than a mess. I don't have any experience with rails, so I can't say whether the mess would be worse, but those engineers are definitely going to make one without guidance.
Nobody understands the conventions so they cargo cult whatever they found from a blog that was last updated in 2013. Or they find novel and extraordinary solutions to problems that the framework literally already solved. Or they have zero experience modeling things in a database—much less doing it the way ActiveRecord encourages—so every battle is always uphill.
The biggest problem with convention over configuration is when you have a team where nobody understands the conventions. But what else do you expect?