A game engine is only as powerful as the developer or the artist that uses it. I have seen some games that used a expensive engine with subpar results. The quality of the game comes from the gameplay and aesthetic both of which aren't really engine bound elements.
For a low budget game they are going to want to prioritize where they spend there development time. The Unity engine is perfect for the types of games were you don't need to reinvent the wheel or advance the state of the art.