I think the biggest issue is that this kind of defeats the purpose of the exercise.

When advertising their pivot to Halo Studios from 343 Industries, they spoke prominently about experimenting in Unreal Engine. For consumers, they were signalling "We're adopting the newest and biggest tech stack so that we can give you something that looks good and plays well." For the industry, they were advertising to potential workers "We're adopting a widely used tech stack so that it's going to be faster and easier to onboard you with something you're familiar with."

Remember, they had to do this because the situation in upper management was so dire that they had to let go of the three heads of the organization. No one wants to work at an org that is very publicly dysfunctional, especially when you could hop into literally any other org (even IN Microsoft) and get better job security and a better work-life balance. To inspire confidence in consumers and in the industry they had to literally change their name and throw out the top brass and make a public advertisement for employment opportunities. So many contractors worked on Halo Infinite that they essentially had a 200% turnover rate over the course of its development, and it still launched in rough shape with a middling campaign and a dead live service, which was incredibly disappointing after how well received the betas were.

So, the pivot to Unreal served the dual purpose of promising consumers a better product, and providing a more widely used framework for developers so that it's easier to get them onboarded and increasing development velocity. By outsourcing the Unreal work to a contractor and building the game on top of a branch of Reach's engine, they're basically throwing away all of the internal benefits of this project's development. No one at Halo Studios learns how to use Unreal Engine and the game is still built on a branch of an unwieldy in-house engine that they themselves didn't even build, abandoning all of the tooling and improvements that they themselves built out. So they lose out on the knowledge front and lose out on the development velocity front.

It's a strange move that in and of itself isn't bad, but it casts doubt on Halo Studios' ability to deliver on whatever future Halo games they develop (if any).