XV's design issues are just that; design issues. Now, it's true that technical problems (which certainly have a habit of cropping up more often when you're moving things onto a new game engine) have an impact on all areas of development, but we've seen throughout the past couple of years that they could never quite decide what they wanted XV to be. This game has been playable to a commercially acceptable degree in Luminous since Episode Duscae; more than good enough for a capable team to work with.

From E3 2013 to Episode Duscae (two versions) to the Platinum Demo to the final release, they've been consistently making big, fundamental changes to the core game mechanics. Responding to player feedback isn't a bad thing, but it still slows everything down. Take the technique system for example - in Episode Duscae techniques like Tempest were things that Noctis had based on the weapon he had equipped. By the final game these had been removed and were instead given to the AI companions. So they had to re-balance the combat system to account for that. They had to re-balance the economy, they had to change the way the weapon upgrade and skill tree worked. Redesign the AI to take the change into account, new user interface and so on and so on.

FFXV is a game full to the brim of 'great ideas' but it's also clear that it never really had a cohesive design until the late stages. Even the systems they have now are horrendously unbalanced in places (magic, items, summons) and that's just an inevitable consequence of not having long enough to playtest a completed system. Engine troubles... yeah, it was probably a factor somewhere along the line. But I don't think anything would have been substantially different regardless of what tech they used under the hood.