I think being able to leverage the less immediately apparent parts of the game to do cool plays is pretty fun. There are certainly limits, and I wouldn't want Sharuum + Sculpting Steel to be a combo deck in my cube, but I kind of find the Fiend Hunter wording to be more of a feature than a bug. You only get bamboozled by it because you mentally shortcut the card's effect, but mechanically it's quite basic and teaches good fundamental rules understanding.
Somewhere in the 130-150 range I think if wotc weren't so sheepish about printing sufficiently appealing ones.
I don't really feel like this belongs here. Sure, having to play around instants increases the cognitive load of things you need to consider, but that's also true of sweepers, discard spells, haste creatures, sorcery speed creature interaction, potential combo enablers, and relative to your own answers, the threats your opponents can use. What is a game of magic if not weighing your options in the face of uncertainty. Instants just make the consequence immediately apparent. Besides, they are the coolest part of magic.
I don't know entirely what this means, and as mentioned I wouldn't recommend having combos that inherently require a deep rules understanding to pull off, but if you have subtle interactions that you might not realize until you're faced with it in a game I think that makes the prospect of drafting more times a lot more appealing. As long as you can reasonably navigate a draft and be opinionated about your picks I don't think your format suffers from having something like Yorion + Animate Dead as a potential combo. Setting aside that Animate Dead in itself is a bit of a complexity nightmare, and I am consciously playing it and LED in spite of it. I have a "complicated" tag I use on cubecobra (when I remember) to try and be conscious of cards I expect to cause rules questions or just take a really long time to read and parse, and on that note, I would like to nominate Read Ahead as one of the all time worst cube mechanics wrt this topic.
Somewhat related, but I went through the effort to remove any mentions of planeswalkers (that I found) in the text of my cards, because my environment doesn't feature them, plus some other things I considered noise like partner text, and a part of me wonders if I'm doing my players a disservice by potentially duping them into thinking the real cards do something else than what they actually do.