Shadow lairs are extremely hard, endgame dungeons that offer, by far, the most challenging environment in the game (especially due to the update). The rewards for completing each SL are materials that build into several armor sets. This is a fairly sound design, but the VAST MAJORITY of these armor sets are, quite literally, BOTTOM TIER.
For example, the last, longest, and perhaps hardest level in the game, Dark Fire Vanaduke, has a material that crafts into the Almirian Crusader set.