I would say a flaps lock is more critical in terms of operating the aircraft. If you only have slats then you will have a very high nose attitude going into the flare with unusual aspect which increases the chances of a tail strike. This manoeuvre would require a small flare to avoid hitting the tail.
You will also have a very high approach speed, slightly higher than if you had a slats lock (on the A320). This could increase the chances of an overrun if incorrect landing technique is used.
The slats locking means the pitch attitude is more sensible with less likelyhood of a tail strike occurring in the flare. The speed will be slightly more sensible as well.
Those are my thoughts.