what is the difference between "réussir à + noun" and "réussir + noun"?

J'ai réussi l'examen. -- I passed the exam.
J'ai réussi à faire tous mes devoirs. -- I was successful in doing all my homework. (a bit literally)

Réussir qqc. (noun) = tu pass something.
Réussir à qqc. (verb) = to be successful at something.

When talking about passing an exam, there is no difference between "réussir à + noun" and "réussir + noun". You can say either réussir un examen or réussir à un examen.

http://bdl.oqlf.gouv.qc.ca/bdl/gabarit_bdl.asp?id=2168

But I think "réussir + noun" can be used in more contexts. For example, to make a successful meal would be "réussir un repas", not "réussir à un repas".