If we focus on the cards that were originally mythics, there aren’t many. Can you name them all?
Mythics were first introduced during the Alara block, but there were no instants among those 35 mythics in that block. The first one was in Zendikar (2009).
Doesn’t actually feel mythic to me, but these were still the early days and the mythics were still trying to find their footing. This is potentially very powerful, so it should have a high rarity, but sideboard cards don’t feel mythic to me.
The next mythic instant came the following year in Worldwake (2010).
Comet Storm has received a lot of flack for being a mythic in the first Modern Masters set, because it didn’t compare very favorably value-wise to the other mythics in the set. It is still a very powerful card, but the effect is basically just a Fireball and even though X-spells are now rares, it doesn’t feel like a mythic despite the upgrade.
It took over two years to see the next one.
Of all these, this definitely had the most impact. Sphinx's Revelation was first printed in Return to Ravnica (2012). It was the defining card in Standard and sees some Modern play. Still, it doesn’t feel that mythic. It is a strong card and you don’t want to see it too often in limited, so in that sense it’s good that it’s above the rare level, but some cards and some lifegain… boooring. (Which it definitely was in Standard, because you would see it everywhere.)
It didn’t take that long for the next one. RTR Blocks second set, Gatecrash (2013) had this very hyped card:
Funnily enough, of the cards thusfar, this feels like the most mythic to me. Not necessarily much more than the three previous ones, but somewhat. There was a lot of interest surrounding this card initially, but it never could find a home. Probably because Supreme Verdict ruled.
Year 2014 brought us the previously Future Sighted Delve for the Sultai (Khans of Tarkir) and included this card:
I’m guessing this was a mythic for limited gameplay. It’s potentially very strong, but again, doesn’t really feel that mythic. Okay, there’s a bunch of tokens for potentially a very small amount of mana, but that doesn’t seem that interesting.
Another card with both Standard and Modern impact. Two damage for three mana isn’t a good rate, but it is colorless, which matters and casting those big eldrazi was par for the course. I guess this does feel mythic to me, but compared to some of the weird concoctions in other card types, it’s just yet another sweeper. The triggered ability does make it at least somewhat more interesting and does deal more damage than red cards usually do.
Finally, from Amonkhet (2017)…
I do love this card, but only on a platonic level. Extra turns probably should be on mythics, especially in red, where it’s not a common ability, even though red can definitely do it. This hasn’t had much of an impact (except for one video where SaffronOlive can’t help laughing when he uses this to mess up some sort of cycling deck).
All in All
It seems hard to design a mythic instant and besides the first and last one, they feel like rare sorceries, which have been pushed by making them instants. Feels like they should stay away from these in the future as well.
The problem with instants is the flexibility, which means that you actually need to make less decisions. Take Sphinx's Revelation. Since you can do it end of turn, you don’t have to commit to it, but you can keep removal and counterspells up instead. It also made Standard very boring by being way too pervasive. I was playing FNMs very actively during this period and I would only play a Monoblack Aggro specifically, because during many of these tournaments I wouldn’t play against any decks besides these. I would often crush them, but those Revelations were always disheartening. It was just an oppressive card even on it’s own and it had a couple of other overly powerful other cards for the deck. All of this could have probably been fixed, if this was only a Sorcery.
Also, from a flavor point of view, mythic spells should be a huge ritual, not something someone just does at the their whim. This means they should be mostly sorceries, of which there are a lot more of (41 to be precise). They often feel a lot more like mythics anyhow.