How good is he defensively ?
He was heavily injured last 2 seasons, does he have any long term issues?
He is quite decent but defending is not his main strength. His pace is helping him greatly in this regard much like Alba. He is rather strong (1.84 cm) for a left/wing back as well. What truly sets him apart is his offensive abilities. He easily beats his opponents on the dribble or due to his pace. Great delivery too. Could even see him play as a CB whenever needed due to his physical presence, pace, passing ability, great technique etc.
As Messi983 mentioned, injuries are probably the only worry.
Anyway I am a fan of his and think that he is the real deal. Just needs the right environment and sufficient playing time (here comes the problem with Alba around) for him to reach that extra level or two which I believe he is well capable of. If I had to make a guess, he will be our starting left back during the World Cup in Qatar as well.
For instance I would take Firpo every day of the week over Mendy.