Right.
The problem with Exploding Dice in L5R is three-fold:
Firstly, exploding dice are, by nature, unstable; they lead to unwieldy probability "curves" that end with an ever-more-unlikely but ever-more-significant spike on the end.
Secondly, the "keep" mechanic has players rolling decent sized dice pools (6 d10s or more) but only getting to hold onto 2-3 of them. Because dice can explode, this turns into an absolute mess. Keep dice are worth 1 - 5 times as much as Rolled dice, with the exact relative value completely dependent on what you already have.
Thirdly, and most significantly, what is kept is not "successes" or "hits" like in other exploding dice games, but rather the entire number. So 6k2 (a basic damage roll) will almost certainly be at least "10", probably make "15", and has reasonable shot at "20", but could be 30, 40, or higher. A starting character has 34 hit points and 4 "negative" hit points.
World of Darkness has an exploding dice pool system, but they only count successes and the game mechanics do not punish failure nearly as harshly.
World of Darkness is lethal because the power curve is (purposely) tuned to be very shallow, so even a basic NPC can pose a threat to a very experienced and powerful character. L5R has a rather healthy power curve (not as rapid as D&D or Shadowrun, but still significant), yet master warriors and diplomats can make an almost unrecoverable mistake very easily, and are a constant risk from a single lucky shot.
I love the setting, I like most of the mechanics, but the Roll and Keep system is too unstable, too unreliable, and too punishing of failure.
But like you said, this is the 4th edition of the game, one where they completely rewrote all the rules from the ground up, and they kept Roll and Keep. It's nto going away.
