Melee have it much harder, hands down. I play both, and it's much more of a PITA to play melee.
1) Melee pretty much have to stack all the time. This is bad when a boss does a targeted AoE on the ground. If it's cast at a melee, all melee are in it.
2) Any AoE bosses do (whirlwinds, stomps, etc) will hit melee if they don't run out. Once out, they have to run back to the boss, losing on-target time.
3) Melee have to be positioned behind bosses at all times. Sometimes bosses have a spot behind them where you can't stand, so positioning can be somewhat difficult.
4) The only thing melee can see during a fight is the huge dragon taking up their whole screen.
5) When a boss needs to be tanked at a certain spot, and the boss just happens to put a void zone/fire/whatever right behind him, melee can't attack.
Ranged don't have to deal with any of these things.
1) Ranged can spread out or stack based on the mechanics of the fight.
2) Ranged don't have to worry about proximity AoE at all.
3) Ranged don't have any spot they need to attack the boss from. Generally, if you aren't in fire, you're good to DPS.
4) Ranged can see the whole arena that the fight is taking place in. Ranged can see things as they happen and react proactively.
5) If a boss puts a void zone on the ground where ranged is standing, they just take a few steps and continue to DPS.