Bursts can be punished, while there is no detriment to using Xfactor.
There is no advantage to be gained from being hit in BB, expending a valuable tool to obscure your mistake or mis-read does not leave you in a + position. Not getting hit is a bit better than getting hit and throwing one of your few defensive options out the window.
On the other side, Xfactor can be used while under pressure, is neigh nonpunishable on activation, and more often then not can lead to an easy OCV. As game mechanics go, in my opinion, it's definitely one of the strongest options existent in a FG.
Expending a burst at best leaves you back at neutral, maybe slightly + for a more defensive character, while Xfactor's level of gain is inarguable.