If you have a 10 person team and your game makes $10M after roughly 2 years dev, with Unity you spend about $2,036,000 but with unreal you pay $2,500,000, just taking development and licensing costs into account.

If unreal gave you 20% more productivity due to being more stable, I think even successful games come out ahead by being able to release sooner (saving money on dev time), or by better sales from having a more stable and visually appealing end result.

Unity is costing me days per week right now fighting with flickering terrains or line renderers that throw errors, all stuff that ends up in the ever growing "known issues" list