Did the math on the Undaunted Pledge Master coffers. Sharing the gist in case it saves you keys.

Quick conclusions:

• Glirion and Maj al-Ragath each sell 6 sets. For both of them, mystery is always better. Doesn't matter if you have 1 unknown or 36 - the 8 mystery pulls you can do for the cost of one curated will beat that one curated on expected new pieces. Reason: curated still rolls one random piece from the 6 in the set, so it's only a 1-in-6 shot at any specific piece you want. 8 mystery pulls out of the 36-piece pool gives you more shots overall.
• Urgarlag is the weird one. He sells 17 sets, so the total pool is 102 pieces. Mystery still wins normally, but if a big chunk of your unknowns happens to be in a single set, curated of that set finally pulls ahead. Roughly, once one set holds more than ~half of what you're missing, curated wins.
• Also calculated how many keys you'd need total to clear a vendor doing only mystery: ~150 for Glirion or Maj, ~531 for Urgarlag (coupon collector problem if anyone remembers undergrad stats).
• Curated feels like it should be the smarter buy because you're picking the set. But you're still only getting one random piece out of 6. For the same 8 keys, 8 mystery pulls just gives more chances overall.

Full math, with formulas: https://rudnkh.me/notes/eso-undaunted-coffer-math/

Calculator (pick the vendor, plug in what you still need, get a verdict): https://rudnkh.me/tools/eso-coffer-calculator/