Echohack says that their research revealed that after six legendary item drops in a day, your future drop rate severely decreases. The same applies to normal gems awarded when playing in a four player group – the first six found each day have a much higher drop rate, dropping to an estimated one normal gem per hour afterwards. In addition, side quests, purple bosses, cursed chests, and random map events all stop giving rewards after the first five in a day, while Library of Zoltan Kule treasure rooms are similarly limited to five per day.
Hidden lair dungeons also stop giving gem rewards after the first six normal gems dropped, meaning “you can only do one, maybe two layers a day before they become extremely worthless.” Echohack also addresses the gathering marks used to progress shadows, which are an important part of endgame. They note that it’s difficult to track how this number is calculated, but state that its progression is limited to 10,000 per day with any additional marks submitted by a player being worthless.
As these caps are not mentioned by the game, you’ll have to manually keep track of how many rewards you’ve collected from each source if you want to avoid wasting your grinding time. Some caps are displayed, such as the restriction on paragon level xp earned in a single day – which is directly tied into your blacksmith upgrade capacity. Echohack notes that while limiting paragon xp isn’t a problem by itself, tying it into other mechanics creates an artificial restriction on those systems.