Erratum: This card’s [second] ability should read: “Discard the top card of your deck. (Limit once per phase.)” - FAQ, v.1.4, September 2018
Q: Can you use Yaotl to discard cards during the mythos phase, or outside of any player's turn? If so, is there any limit to how many times you can trigger Yaotl's ability (i.e. is there any limit to how many cards you can discard outside of players' turns)? A: If a card has a limit which specifies a particular period of time (such as a phase or a turn), it can only be activated during that period. For example, if a card had a limit of “once per phase,” then it can only be activated during a phase, and not outside of a phase or in between two phases. In the case of Yaotl, his ability can only be activated during an investigator’s turn. <- [NB Answer out of date in line of erratum, above.]
Q: How does Yaotl work in combination with Steadfast, Curiosity, Cunning, Prophesy or Able Bodied, or other skill cards that gain icons while committed to tests? A: The abilities on skill cards are only active while they are being committed to a skill test, unless otherwise specified. In this case the skill cards will only have their printed icons while they are in your discard pile, hand, or any other state other than while they are being committed to a skill test. (This includes an effect that says something like "discard a card from your hand with at least X icons...")
I think a good use of this card could be with Agnes Baker or on another level with Patrice Hathaway. Combined with Scrying or Scrying(3), you can see 3 cards of your deck, and decide to discard the one you want, especially Treachery Cards. Moreover, correct me if I'm wrong, but Patrice Hathaway can choose the order of the cards in her discarded hand, making Yaotl sensibly more powerful.
Recurring 4 icons? Yes please! I found that between committing the card, then using Yaotl, and then using Yaotl again at the start of the next turn, I was using the same card 3 times - and it was pretty easy to do so! That is pretty incredible.
The second free triggered action "Discard the top card of your deck" - this is useful if you end up with a non-iconned card, like a weakness, on top of your discard pile. You could pull the next card from your deck and hope it has icons you can use. However, I didn't actually use this - I didn't want to risk discarding those Desperate skills.
So, it can work nicely in the right deck ... but generally, I suspect Yaotl is a bit "meh". You want a deck with lots of icons, and that probably means a skills-heavy built - and there aren't so many of those. Also, most cards don't have more than 2 icons. It's quite a limited user-space.
Calvin Wright - You don't want lots of icons, so Yaotl doesn't seem to work well with Calvin, the other obvious candidate for a Desperate skills deck, given that Rise to the Occasion seems made for Calvin. Yaotl might work here.
Minh Thi Phan - Seems good for a skills heavy deck, so Yaotl might be useful here. With a little creativity about who you help out, you might be able to make sure you've got the 'right' card at the top of your discard pile for the start of your turn. Yaotl might work here, but is up against great Seeker allies. If you're using Dark Horse in Minh then you're probably not using Dr. Milan Christopher, so in that case I'd give Yoatl a look.
Wendy Adams - Might have some interesting synergies between her abililty and then being able to use the icons on the card discarded. However, I imagine our little evade-tank will want Peter Sylvestre more.
- William Yorick - Probably has more combat-focused allies to put in play.
Otherwise, it's hard to see how Yoatl can displace other allies for other investigators.
I guess this could also work nicely with True Survivor, to haul skills back out of your discard to reuse again.
In short - a really useful tool in the right deck, but possibly rather specialised for general use.
(Though I might have that wrong and people have lots of unusual builds that work with him...)