Many open-source projects simply don't have the resources to adequately test their products or provide support. Contrast this with a large company which has the resources and the willpower to provide support for their software. Often the best of both worlds is a large company/organization that dedicates its resources to an open-source product, but that's not always the case.
But this issue is never as black and white as "open-source is more secure." There are many other factors that go into the security of a product beyond its source code being readable. Deciding which factors matter largely depends upon your unique threat model.
But this issue is never as black and white as "open-source is more secure." There are many other factors that go into the security of a product beyond its source code being readable. Deciding which factors matter largely depends upon your unique threat model.