As a developer, pretty much every proprietary engine I have ever used has ultimately been terrible. When you develop in-house, you run into a lot of issues with engineering resources.
See, when you license an engine, you are also getting support and tools. You get updates to the engine, tools patches, documentation, etc. You aren't just given an engine, you get the entire support structure whatever company you are licensing from has (both DICE - Frostbite and EPIC - Unreal are really, really good at this).
But in-house, you are at the mercy of the bandwidth of your engineering team. Because most of the time the engineers who built the engine will either ... leave the company because they're engine developers, not game developers; be reassigned to different engineering tasks, dramatically limiting the time they can assist with engine-related issues like tools; or put full-time on engine optimization since those licensed engines have multiple years and 100 engineers working on the same problems and you need to be able to compete with them performance-wise.
So you end up with busted or inefficient tools that make development take much, much longer than it needs to (see: Destiny), the potential that undocumented engine quirks and features get lost forever because the engineer in responsible for it either got laid off, quit, or reassigned, and limited engineering resources since at least some will be in a constant state of optimizations right up until launch (and potentially after for service-oriented games).
It sucks. I wish every in-house engine project I've worked on would have just licensed something instead. It's rarely even cost-effective anymore to develop on an in-house engine. Most studios only continue to use their proprietary engines because of habit (all the devs know their own engine) and because of sunk costs of originally developing it.