Yeah EX low fireball gives you a free jump on hit. I wouldn't call it a rune trap, because rune trap is godlike and unblockable. This set up is avoidable.. low fireball hits low yes, and yes that is good. It can also be thrown after 21 which gives a mix up at the end between the low and the overhead launching 3. That's the good, this is the bad:
-2 in the 213 string is a high and can be ducked by all characters, regardless of whether or not they are blocking. If they are ducking and not blocking, some characters can probably take a free uppercut on you. Sonya, Quan Chi, Nightwolf.. If they can't, they will definately be able to D1 or D3 poke.
-If your low fireball mix up is revealed once, or if they know it existed beforehand, they can block low and look for the overhead 3 to come out on screen, react to it, and block it before it hits them. Compared to most, this overhead comes out quickly, but it is possble to read the difference between it and a fireball.
-If your opponent guards you successfully, you have used a bar of meter and gotten only chip damage for your trouble.
My most used string with Kang is F1 2. If for some reason they're not blocking low during my F1 2, and I have good meter to spend, I'll add the EX low fireball on at the end and take the free jump in mix up to either F3... B3 1.... F1 2 again... or jump kick throw. All four options are pretty safe, so unless it will kill them or close to it, don't go for the throw.