Both advantages and disadvantages, and in competitive battling.
If you're a Physical Attacker and the opponent is a super annoying Physical wall with Defense Boosts. You'll have to Force Switch - which can be costly. Whereas if you are a Mixed you can target the weaker SpD Side.
Some Pokemon have limited Movepools to choose from.
If you have great Atk and O.k SpA, but hardly any Physical Moves and a neat range of Special ones then you'll have to be a Mixed Sweeper or just be really unpopular in the Metagame.