I haven't posted here for a little while (decade/s?) but here goes - this topic looks worthwhile rolling around in the head.
For me there is an intermeshing of a few different contexts. Complexity can come from a process or procedure being unfamiliar, or it can be because the process is finicky, requires a few interactive loops, or possibly generates a result that is indirect, or unaligned with the narrative. I think it was Monte Cook who described RPGs as: the best hour of fun you can have across four hours... or something similar. If you like, complexity at the table is more often in that three hour component, than the one hour.
And so the question becomes what complexity in a games becomes part of the fun hour? Complexity is really about time at the table.
I think this is very much related to how time is being treated in-game - what frame or mode of play is being used.
1) If it's much real time: 6 seconds gametime - in other words encounters or combats where everything slows right down - then you want to keep complexity out of this, or at least have a lower tolerance. If three players are using their "paper buttons" and achieving an in-game result with minimal adjudication or GM interference, then this keeps things flowing in an environment where your turn is only one amongst the many. If the fourth player is negotiating or playing mother-may-I with a more complicated ability or process that colours outside the lines, this is where complexity bites.
2) If it's 6 seconds real time: 6 seconds gametime, such as in scene or "roleplaying" with NPCs, then this is where you want ZERO complexity. You don't want mechanics pressuring the players out of this mode of play - you want it decided by the narrative.
3) If it's 6 seconds real time: much gametime, such as exploration, through to montages, then to "downtime", then I think this is where you can spend most of your complexity budget. I think the two you mention (magical rituals and crafting) are excellent examples. Personally, I would differentiate between rituals (one exploration "turn" up to an hour), works of magic (several hours to possibly several days - or ongoing), and crafting permanent magic (several days to several decades). However, as has been seen in this thread, this is player/game dependent. An OSR crew will most likely be in their happy place if they have to manage intricate resources and follow smart procedures to survive. Others just need to hear the word encumbrance and they're up and away from the table. If a complex procedure vibes with the style of game - then it's most likely worth it as it's the type of game the players have already bought into.
For myself, I like complexity with certain exploration activities. Disarming traps, unlocking strange locks/devices and anything where focusing on the in-world stuff is important (and can't be procedurized into let's do macro one for this, or macro two for that). I would love more complexity with healing wounds - treatment, surgery, and so on as it fits in with a grittier game (rather than a hand-waivy heroic style). I would love tracking to be more involved than a simple check. Discerning, identifying and understanding magics and spellcraft deserves far greater complexity. Haggling or bartering as a more complex procedure. For me, such things are juice worth the squeezing.
