Gotta be careful with on-rails touchscreen games where you have to split swipes between yourself and the enemy. I tried out Lunar Knights a little while ago, and they have sections like Kingdom Hearts' gummy ships. The problem is that you can't dodge incoming bullets and retaliate at the same time, and sometimes the fat ass of your ship will block the target you mean to click on -- meaning that it's easier to shoot at a funky angle than to shoot somebody who's literally right in front of you >_>

Ideally I'd have some sort of jump button totally separate from the touch-to-fire system to make it kinda like a Canabalt with guns. If not, you'll have to very carefully balance your game to make sure the switch between offense and defense/evasion is practical. You don't want enemies who fill the screen with projectiles you have to dodge, then force you to shoot them to make them stop firing in the middle of their salvo. Because offense or defense will take a hit.

There are quite a few games with this mechanics on iOS. To name a few, Jetpack Joyride, Temple Run, Agent Dash, Chasing Yello, Rushing Alice, Tiny Wings, Ski Safari. And there are others I can't recall the name, the one with gingerbread man, and the one with birds.

Most of them are free, with in-app-purchase based power-ups. And the operations typically involves jump and dunk only. And for Jetpack Joyride, Ski Safari, Tiny Wings, they only involves one operation. It's not typical to have something to "shoot at".

You can also consult Doodle Jump, which is constantly moving, but at controllable progress.

In the end, you are asking whether it's a good idea. I have to say there are already many hits in this genre. So people are familiar with it. How they are designed, I have to agree with @slicer4ever on pacing. Some games are bad because they are too slow or fast. But you have to play the existing hits to get the idea.