Encounter design being one of the strenghts of Larian and verticality already being present in their previous games - and also in D&D -, it would be quite a surprise if it actualy upset the balance of the game or make most fights too similar.
But if it does, players will see it in early access, there will be all sorts of heat maps to show it to Larian (as seen in a few interviews and presentations), and it will likely be adjusted one way or the other.
Larian probably won't limit weapon range as much as they did in DOS2 since it would go against the core material, and to an extent against the spirit of D&D. So sniping will probably be posible, but it should advantage the player. If I remember what we've seen about sneaking correctly, enemies will have a line of sight, so they seem unlikely to take down players from far out unless an encounter is designed for it.
I'm mostly curious to see how it will affect the design of the Underdark. We've seen glimpses and it looks like a relatively complex and layered open environment where verticality might make encounters more complex to design and balance.
