Eternal Analysis 014—Ambush and fast spells

Eternal avoided making Magic's mistake of taxonomically dividing sorceries and instants: They're all spells, but some are fast and some aren't. Then they dropped the ball.

Eternal hasn't made any other types fast. There's one recent card that makes your weapons fast, but that's it. There's not a desperate need for fast relics or fast sites, but a naturally fast weapon makes good sense, as might a fast curse. I'd expect 1 or 2 fast non-spells in a CCG now up to 1721 cards. What about fast units? Well—

There are 16 units with ambush. Ambush is a skill that lets you play a card out of turn. It's nearly identical to fast spells, except cards with ambush can only be played after your opponent declares attacks or ends their turn (where fast cards can be played in response to most spells or effects [I say 'most' because after 6 months I'm still not sure where the line is]).

If I were remaking Magic and had already won the victory of consolidating sorceries and instants into spells and fast spells, I would most certainly try to continue that streak with fast units. It's a good bet Dire Wolf tried that too, so let's consider what would have forced them to use ambush instead.

Dire Wolf has done a great job reducing the number of obligatory clicks needed to move through a game, and the vast majority of fast units aren't relevant responses to spells. That does prevent Eternal from making Frilled Mystics and reduces the power of a Snapcaster Mage, but your Ambush Vipers, and your Faerie Duelists still work fine. (It is counter-intuitive not being able to play Desert Marshal after you declare blockers on my turn.)

Speeding play is clearly the main benefit, but is it enough? The only other reason I can think has to do with the back-end rules difference between a skill and a card type. Kosul Elite can grant the fast type to a card, but Crown of Possibilities cannot. Rhinarc Huntpack, Agile Deathjaw, and Tumbling Sloth all have ambush conditionally—how would they read with a conditional card type? Actually, they'd read great, but we must grant it's a bit awkward to print a card's type somewhere other than its type line.

I feel like Dire Wolf prioritized speed of play / click reduction over design elegance / cognitive load more than I would have, but without having playtested fast units within the UX, I can't say with authority I wouldn't have made the same choice. I will say they could have eliminated the main difference by simply having fast apply to units differently, still limiting when you can play a fast unit to when you could play an ambush unit. It's not like they don't have other facets that work differently depending on context.