I'm not certain what you were told, but the root issue isn't anything like corner radius, but rather how visual effect view works, and how masking impacts that functionality.
UIVisualEffectView works by capturing the content behind it, and then applying filters (such as blur) to that content. This is important to understand because when visual effect view goes to do that capture, it can only capture what is in the current render buffer. But when you do various tasks that require an offscreen pass (such as masking) that creates a new render buffer that then causes the capture phase to capture less content – only what is in that current render buffer.
So if you apply a mask to a UIVisualEffectView via the CALayer.mask property, you end up putting the whole visual effect view into an offscreen pass, which means it cannot capture the content it needs to render. If you use the UIView.maskView UIVisualEffectView works around this by taking a snapshot of the mask instead, and applying it in the correct places to ensure that the capture can still do the correct thing.
One bug that we fixed recently had to do with snapshots of that maskView not working – that bug exists in Beta3.
That is why it is important to use the UIView.maskView property when working with a UIVisualEffectView instead of the CALayer.mask property, as otherwise the effect will be broken. Other bugs allowed this to work prior to iOS 10 in limited circumstances, but usually by causing incorrect rendering.
Now corner radius came up with one of the reported issues because the developer was basically just using a rounded rectangle mask – cornerRadius+clipsToBounds does not have the issue that general masking does, so that would work without any effort. For most of your masking needs, I suspect the advice is irrelevant.
I would also highly recommend that anyone with questions on broken visual effects watch the 2015 WWDC session on Whats New in UIKit Dynamics and Visual Effects, where we discuss all the ways your effect can break due to offscreen passes.