I am not against checking out all your options for IVR, in fact, I say do so. I will say, not all IVR solutions are the same, nor do they all have the same capabilities, or ease of use, set up, and user administration ease.
I will say, it seems a bit common sense that a phone system, and IVR developed by the same company, as part of the same system platform is going to have some advantages over just picking one off the shelf. Also, when it comes to support, there is no ability to point fingers at the other tech, product, or local company servicing the product, or products. It is one product, one manufacturer, one tech, one local service providor company. With experience based on your platform specifically. After considering those tings, the playing field is pretty much equal in other regards.
You do not always get what you pay for, but you never get what you do not pay for.