Tailwind operations increase risks above those for headwind operations (takeoff and landing); most are countered by general certification and operational requirements.
However, the aircraft certification requirements for tailwinds greater than 10kts require special clearance*; where granted they may be associated with specific limitations / guidance. Those which I have seen and used, were operator / location specific and should not be read across to other situations or operators without careful consideration. If it isn’t in the AFM, the operation is not certificated irrespective of ‘an ops manual’.
The aircraft certification will validate the aircraft handling characteristics, including engines; for landing that the aircraft can be flown accurately to the required touchdown point in the higher winds, and that systems operation is not compromised; e.g. squat switching and retarding devices. etc. This might not consider all cross wind / turbulence conditions.
The landing distance performance assessment (RTO perf also) might not be so rigorous. The normal flare time and flight characteristics might be used with simple extrapolation of distance with speed (GS).
However, in operations the ideal touchdown position might not be achieved as consistently as with head winds, due to the tendency to extend the flare / float, which has a significant impact on distance – very much more than a linear interpretation. Similarly any delay in using the retarding devices will have a large effect (N.B. RTOs).
Also consider the reasons for landing downwind and any ‘external hazards’. IIRC one night-time noise abatement procedure allowed 15kts tailwind, but the risk of overrun increased significantly as the majority of braking was on the portion of the runway used normally for touchdown and was heavily contaminated with rubber deposits – not ‘safe’ when wet.
* FAA AC 25-7 Flight Test Guide For Certification Of Transport Category Airplanes
Tailwind Takeoff and Landing - Wind Velocities Greater than 10 Knots.