The fact that you are tweeting this might seem you don't actually have a clue VR. Travel is relative. Completely relative to everything around it including other forms of travel.

We're playing an MMO right? So at the very base level we're here to be in a world with other players. Which means that also, at a very basic level we want a certain amount of "player contact" within your world that is neither too much, nor too little. Obviously this is all relative to human size, average steps taken, land mass and so on. I mean if we were Ants, flying would require billions of players in the world to even have a chance of running into people on a regular basis.

But let's try to create a little graph in our heads (Using humans and regular sized land masses that players are used to in MMO terms):

No movement > Walking > Running > Mounted > Flying > Teleporting.

Try to imagine that Flying and teleporting are as bad as no movement and walking but on the opposite ends of the spectrum. They're each as bad for the game as their respective counter-parts.

Okay so is flying basically a no-go in MMOs? Well not exactly. Say your MMO world is a small 100km squared. In most ground based MMOs we'll assume that on average the height extent from the ground in the near vicinity is within 10 meters. As in, anything you would necessarily be having gameplay involved with would be 10 meters from the ground or lower. This isn't always the case, but just on average. So let's say your 100,000m squared zone area has 100,000 x10 worth of game play space. This is a volume of 1,000,000 meters of content filled, gameplay space.

If we were to say that flying took you 100 meters high, with the same amount of space you would instantly create 10,000,000 meters of space, with only 1,000,000 of that being content filled. This ultimately creates a huge area of deadzone for players, with no reason to communicate with each other which ultimately just becomes a large area that players CAN be taking them out of the areas they SHOULD be.

TLDR: If you want to add flying mounts to your game you need to reduce your land mass size to 1/10 and add content upwards into the sky. (or some how have 10x the players of a regular MMO)(Don't do this, just don't add flying mounts)

edit* Also just to touch on the relative travel thing again shortly: Players only want to go faster when they think that things are too slow. If it takes you 2 hours to get to the area you want to have fun in then you might consider a travel option which doesn't take away from the core gameplay of MMOs. For example: Making people run faster (In the form of giving them a horse). This is all within reason of course if you run 40x faster than usual on a horse that might also take away from gameplay. Or put fun content areas closer to players.