It caused them headaches because it was still in development. The engine was developed alongside the game. I wouldn't even say it even caused them headaches. They seemed to have a working engine in place for a while, it was just optimizing for consoles that became the main issue. If you have realistic art direction under the specifications of your target platform from the beginning (something FFXV wasn't able to do) you could easily create a game that runs very well with Luminous Engine. XV is evidence that even as the first game using this engine, it runs with great performance pretty much 90% of the time.