Don't know about these, I suppose they wouldn't be too bad as options for some players, but I certainly wouldn't want them to be integrated into the core.
It might be just a matter of preference, but in games like Daggerfall where you can reload at any time, chance based actions usually just lead to save scumming. The only exception I've seen is players who are very heavily role-playing the character, but even then, it more often leads to frustration and occasionaly truly fun moments, but these are rare. Chances work better in games like Darkest Dungeon or XCOM where you are forced to live with results of your actions and can't just reload a quicksave. With something as powerful as spells, this will only be more severe, where you load the same save dozen of times before finally succeeding with the dice roll and hopefully overcoming the challege, otherwise you have to go back to loading.
Imagine yourself trapped in a room with Ancient Lich and having powerfull fireball spell that would probably kill it, but only has 12% chance of success and no other saves from recent past. Even if you don't like save scumming, you'll most likely do it in any case, just not to lose your progress.
Similar problem appears with drawback effects where we can see min-maxing often taken to the extreme as seen with advantages/disadvantages system of Daggerfall. Players will often pick powerful advantages and a ton of minor disadvantages which while affect the game to large extent often feel more restricted than liberating. These are really hard to balance and same plagues are likely to haunt magic drawbacks. On one hand, it can be really fun, but on the other hand, it's hard to resist temptation...
While these features would give players more control over their experience and more fun ways to break the game, I feel that magic system that will try to be both fun and balanced would be greatly appreciated. It's an extremely challenging task, especially if the system is to be flexible and include custom made spells, but it would provide more engaging and compelling experience overall. But that's just my opinion and I certainly wouldn't mind extra options.