Having played neither, I can only express that I don't consider invisible walls in any way or shape a "problem" or "flaw" of an open-world ended game even if it would be desirable not to have them, though that could also create pointless space that contributes nothing to the game. If you consider the gameplay good enough, then that's how you should value it in relation to the open-world aspect.
One of my favourite games ever made is Dragon's Dogma, and...
That's the whole map of the game. See all that dark brown areas? Invisible walls/unreachable space. Didn't really see it as a flaw, especially when people complained already that the map was too big (considering in the original version there was no way for instant-travel aside from the main city and one specific point you'd design, so you would have to backtrack like crazy). Sure, having more map would have been nice, but the game worked fantastic with what it already had.
I haven't played Horizon because I don't really have any hype for that game, but I wouldn't consider map limitations as a flaw, if the developer didn't really find any worthwhile to put there for you to explore.
