Depends on where you take them.
Decently geared A2, A3, and A5 mercs perform very well in fauzt and brem. They do well in dunc, TA and teganze but can occasionally die to shotgun attacks like stacked barrels, 20 teganze balls to the face, and arrowside (mostly a2 and a5. A3 merc rarely dies to this). Can't speak for a1 but I find other mercs very durable in most farming ubers, including vizjun.
Max their res, and get them some PR. Barb merc gains more regen based on Str so stack Str after resists and PR. Also give them some FHR and IAS/FCR.