If all you have to do to target is drag a card (for example Lightning Bolt) onto another card, that's simple enough.

However, if you need to select multiple targets with single taps (for example a fight spell), it is very easy to miss your mark - and there's no way of getting a do-over. Once you've tapped the final target, you're locked in.

Iirc, on PC, you get to submit your choices, but when I cast my Blizzard Brawl in my first game on mobile, the spell resolved immediately after me tapping the second target with my finger.

If that was only an isolated incident with my Blizzard Brawl, please let me know, so I can submit a bug report!