Pica is fine because he fights indirectly so far, since he attacks while being deeply covered by massive amounts of rocks(and to get the original body, it must go through that massive amount of rock before actually touching the original, giving him more than enough time to get out of the way with his near instantaneous movement speed around the rocks), making him close to invincible defensively in this case due to his great staying power, but his combat ability in itself is inferior to Zoro's(at least from what he showed so far) since despite having basically the entire country as his weapon, he never managed to land a hit on him yet
For the Vergo/Zoro thing, in PH Zoro felt Vergo's aura and we know through Rayleigh that with the aura, you can determine one's combat ability, yet he left Sanji deal with him. He never gave me the impression that he thought Vergo was an opponent impossible to overcome