What can go wrong?

Very little (with either technique). But it is important to understand that with any surgery there are risks involved, even if the surgeon does nothing wrong and all goes well.
In some cases, individuals can be under or over correction. About 95% of patients have no complications (whatever).

Minor complications
which usually recover completely, including raised pressure, temporary water-logging in the front of the eye (corneal edema) or in the back of the eye (macular edema), and a tendency for the upper eyelid to droop slightly (ptosis), or bleeding inside the eye.
One of these rarely causes permanent effects.

Serious complications
would include retinal detachment (1% of cases) and infection within the eye (endophthalmitis - approximately in 1 in 1,000 cases).
Both of these conditions are treatable if detected early enough, but this may result in permanent and severe visual loss, including blindness or loss of the eye (very rare).
It is important to understand that the risk of these complications is intrinsic to the surgery itself, and can occur even if the operation is performed well by an experienced surgeon.
The occurrence of one or more of these complications does not mean that the operation was poorly performed.