You are right in that ablative lasers usually have better results per treatment session. An Asian study by South Korean doctors did a comparison of lasers for acne scar treatments and found that the non-ablative (i.e fractional laser) treatment groups had a slightly lower improvement score and required a greater number of treatments compared to the ablative laser treatment groups.
However, patients who had the ablative laser had a prolonged recovery period and a higher risk of complications. Complications of ablative laser include hyperpigmentation, hypopigmentation, skin infections, prolonged redness and laser burns. It is for this reason that many doctors may advise you to start off with a fractional laser instead.
The study also concluded that 3 consecutive fractional lasers are as effective as a single treatment of ablative laser. This may be a good trade-off, for a much lower risk of complications, especially in Asian skin, which is prone to developing hyperpigmentation.
