The maximum advisory (demonstrated) crosswind value should be used with caution. A truly limiting crosswind value would be in the AFM limitations section, but even this might require qualification.
Max demonstrated (and limit) values provide the best advice based on the manufacturers tests; but only the crew can apply this advice to a given situation.
Thus, crews need some understanding of how the values are obtained, and greater understanding of the conditions which can affect how the value should be used.
Manufacturers usually have high accuracy measurements for the wind; these are rarely available in normal operations. If ATC reports gusts there may be few indications of a direction change and perhaps none for the duration of the gust. Pilots require gust direction, magnitude, and duration for a complete understanding.
Flight tests may be conducted on a range of runway surfaces, but these are rarely stated in the manuals. Thus, there could be a considerable difference in crosswind capability on a wet grooved or high friction course surface, and on an un-grooved wet concrete runway; the latter can be very unforgiving. Crosswind landings aren’t just about aligning with the runway, they include staying on the runway.
Similarly how wet is wet. Reported good braking in wet conditions is more often associated with ‘light rain’ and/or good drainage, thus in moderate or heavy rain, or even light rain with poorly drained runway, the braking performance could be distinctly ‘poor’ and thus reduced lateral control. There is no sharp transition between wet runway and a flooded / contaminated runway (3 mm). Thus anything between wet (25 kts xwnd) and contaminated (5 kts xwnd) has to be judged and carefully considered together with the runway surface and wind measurement factors, - note that the manufacturer might not have tested these combinations and only used computer models.
Also, consider how the braking action is reported. Other pilots using thrust reverse, and/or autobrake on a ‘poor’ runway may provide an unrealistically optimistic assessment.
Manufacturers’ limits and advice should be based on the skills of an ‘average’ pilot; however due to the difficulty in judging a world-wide ‘average’, workload is often used in the assessment – tolerably low workload. Occasionally, in addition to the max demonstrated value further guidance (lesser values) may be provided for a range of situations; these values may also be based on simulations.
Thus judgement of the level and currency of skill required for a crosswind landing remains with the operator or more often the Captain. A human weakness is a tendency to judge capability as being better than it really is, so some safety margin should be provided in the judgement. Consider how often crosswind landings are practiced, what maximum value has been experienced recently (and what were the conditions), and has the limiting condition ever been flown ‘for real’.
Also, consider that for most other limit ‘speed’ situations, aircraft certification provides a safety margin – stall margin, Vmo over speed, flap / gear limits, which can accommodate operating error. No margins are required when certificating crosswinds, yet these situations involve many opportunities for error.
Thus a safety margin should be added to the crosswind wind ‘limit’, gusts and turbulence should be considered seriously, and as being within the limit even if not published. Start with the lower values.