1) Tab targetting is unreliable for a number of well-documented reasons. The first tab won't move off the current target; if the mobs move around, the tab order starts over; tab distance is uncontrolled; etc.
2) Click-targetting is even less reliable as health bars dance around in a jig.
3) Mouse over targetting is only reliable if you have a tool-tip add-on (Living Bomb Tracker, revisited) that tells you if NT is applied and how much time remains. If parts or most of a potential target are covered up, you will never DoT it.
You need to target a mob, directly or indirectly, get feedback from an addon to tell if you should apply your bomb, apply your bomb, all within one GCD. With 180 ms latency, it takes nearly 700 ms for my screen to show me if the mob should be (re)dotted. If it shouldn't be, I've just lost 700 ms, never to be recovered, as I am working well into the next GCD.
All of this is happening while Ignite is falling off, and you are losing DPS on your main target.
No, I am not impressed with what is happening to my mage. I'm being sat for progression in my guild because others can do the same DPS as me, plus they can damage on the move or interrupt more often or provide supplemental healing or mana return, or in some cases, all of the above.