Second, critical hits are fractional rolls are triggered off advantage, not triumph.
No, a triumph can trigger a crit. The purple die is weighted more to threat than the yellow is to advantage, and if you roll a triumph there is one of your yellow that is not contributing to advantage. Triumph will usually be used to trigger the crit, if you do have enough advantage to trigger then it can upgrade the crit by +10. Both will have far greater effect than a knockdown.
The fact that you get triumph only 1 in 12 on a yellow ALONE makes the talent not worth taking. That it will often be the choice between knocking down or the superior choice of doing a crit makes it a very bad choice of talent. As I said, knocking someone out is superior to knocking them down, making them unable to take an extra maneuver for the rest of combat is superior to giving them the choice of spending a maneuver to stand up once.