Rubi is just rule based integration. So its like looking up a book of integral formulas and encoding them as rules. It does mean you need a minimum number of other features like partial fraction decomposition, polynomial factoring so it demonstrates some capability. Many of the other Mathematica like CAS end up using the Rubi rules themselves.

But indefinite integration is just a small aspect of CAS capabilities. What about integration over a line or surface, definite integration and dealing with singularities, differential equations, solving equations under assumptions, simplifying equations.