It's arguably no more a "hack" than TLS is one. Right?

Re OpenVPN vs IKEv2/IPSec, this IVPN FAQ seems accurate.[0] But then, I helped edit it, so I'm biased. Still, if anyone can point to inaccuracies, I'll recommend fixing them :) The major weakness is pre-shared IKE keys.

On the other hand, I get from IVPN that the IPSEC implementation in iOS is very secure.

0) https://www.ivpn.net/knowledgebase/160/Is-using-L2TPorIPSec-...

Don't see why you're getting downvoted. From a user standpoint, IKEv2 doesn't require a secondary client and integrates with most major OS better.

For example: It's way easier for a client to install a mobileconfig to ios that supports on demand VPN than it is to have them download and configure openvpn. Fairly set and forget.

OpenVPN protocol is sorta weird (I wrote a clean room client and server impl). But IPSec stuff is such a pain to deal with that it is not worth it despite it having better OS integration.