Tbf, I personally believe that it is well balanced right now.
Its always been that some people might think that one location is better then the other. But with the recent changes done to spawning, this era should be gone. Spawning has and will always be RNG based and heavily luck based. No matter the tier, you can get a pokemon after 5 steps or a million. People always tend to think that "tier 6" is easier then "tier 9" because its lower, which means "better". Which may be why it was changed to "Common, Uncommon and Rare", to take the focus off the rarity level. But this is a whole different topic, so I'll avoid crossing into that.
So to get into the spawns a bit. Sure, make some rare spawns easier to balance it out evenly to a point that it won't plummet PRO's economy.
However, doing so would require a finess to make sure that the economy wouldn't plummet because a event xmas poke is easier to catch on the event map(s) compared to other locations. I'm sure the Spawn Editors are doing their best to tweak spawns in favor of PRO's economy and playerbase alike and that they'll appriciate any feedback that players give regarding this.
TL;DR Spawns are currently well balanced, but can be tweaked. But not to a point that a xmas event poke is easier to catch on the event island compared to other locations.