I tend to find myself "jumping into a fight" quite often. I feel it's the only safe time to really purposefully jump in a fight reliably. Jumping the whole fight is like block hitting in an improper situation: The "double damage" aspect makes it sound really appealing, but the huge lack of mobility you have makes you really easy to push around. The way you take knockback damage tends to cause you to take extra hits a lot, which pretty much negates the effect you got from the criticals, especially if the opponent is "jumping back into the fight" for criticals often.
I find jumping into a fight is quite effective, as long as you don't go straight at the opponent. Jump to the side when doing it. It's quite uncommon it backfire, unless the opponent sees it coming, which if you make yourself unpredictable enough, would be nearly impossible to do reliably. This is even if they know how to in the first place.