All security mechanisms feature a trade-off between usuability and security and I don't think that FaceID will be any different in this regard.
One of Apple's original motivations in introducing TouchID was to get iOS users who had no PIN code set at all, to use some form of security mechanism on their device. This would obviously only work if the mechanism had good usuability, which TouchID does.
As to the specific threat you mention about creation of models, from technical information revealed by Apple so far, they appear to be using 3D mapping techniques combined with motion detection to identify the user (there has been mention that it won't work when your eyes are closed, for example).
So to copy that a full 3D model of the user would be needed and that model would need to emulate the users liveness to some degree.
So that's obviously a theoretical possibility, but the question is what level of effort would be needed for an attacker to pull this off, and whether that's a reasonable security/usuability trade-off for most iPhone users.
I'd suggest that if we're at "create an accurate 3D animatronic head to crack" that's not an unreasonable level of security for most users, if it has good usability (to quote @avid's rule of security "Security at the expense of usability, comes at the expense of security.")
If however when it's released, it turns out there's some simpler way of faking it out, that might be more of a concern.
Having seen Windows Hello in action, I'll be more interested to see what the False Rejection Rate is like with this. Hello is great in fairly static settings, but not so good outdoors with things like hats and sunglasses coming into play.