How risky is an afib cardiac ablation on a 33 yr old with no heart disease or other significant problems other than a cspine herniation. ?

Not very: >99% of patients who have ablation performed with have no serious complications. 80% will have lasting relief and the rest may need to have the procedure repeated and/or take medication after the ablation.

See below: If you are otherwise healthy and young the risks are low. Obviously you have significant cardiac rhythm abnormalities.

Small Risks.: In general it is very low risk; however, atrial fibrillation ablation tends to carry some of the highest risk of any the catheter ablations. The worldwide data shows that af ablations have a risk of death of 0.15%, tamponade 1.3%, pseudoaneursym 0.9%, stroke 0.2%, TIA 0.7%, pv stenosis 0.3, and ae-fistula 0.04%. There is a small risk of pacemaker in some types as well.

Not very: You haven't specified what type of ablation you're planning on having, but generally speaking even for the most riskier procedures, with no other health problems, the overall risk of serious complications is 4% or less.