IMHO the biggest con is unpredictability - if you try bigger mobs, the damage you receive can fluctuate a lot. Paladin has more protection than pixie against atrox for example (all 3 melee dmg types). So you can receive 10 dmg at first hit and 40 after that. Those are not real numbers, but just an example. On the first it hits paladin, but on the second it hits pixie part. This can become even more extreme with critical hits, bigger armours, bigger mobs, rare damage types.
Unlike in other MMOs wearing a full set does not give any additional defense or other bonuses.