Naked Man and I are having some interesting discussions right now about critical hits and fumbles in DnD. He favours fumbles existing, and the option of open ended critical hits. I don't prefer either of those, largely because I like difficult fights. To my mind really swingy abilities and mechanics tend to make fights really boring because they need to default to really easy or you wipe out the party regularly.
Think of it like this: Imagine a fight between a hero and a villain who each do 1 or 2 damage every round. If the hero has 10 HP, the villain definitely can't have 9+ HP because the hero is going to die a lot. However, if the villain has 5 HP, they die pretty trivially. However, you can give the villain 6-7 HP and make sure the hero feels threatened but stands a very small chance of being killed. The result of the fight is predictable, but the hero feels like they were in a dangerous situation.
Now imagine instead that the fighters sometimes roll badly and fumble, losing not only that action but also their next action. Also imagine that they score critical hits, enabling them to do a lot of damage sometimes. I model this with damage totals randomly chosen from between -1 and 4. If the hero has that same HP of 10, the villain can easily kill them in 3 hits. Not most of the time, mind you, but it is a real threat, and 4 hits is no problem. Because of this if you want the hero to win you have to give the villain a really low amount of health, say 3 or 4, to make sure that the hero comes out on top. Problem is, that means that the hero is often going to kill the enemy on the first or second turn and will rarely feel threatened.
In DnD fumbles don't occur on every fourth swing of course, and neither do crits. However, the presence of extreme swings in combat really forces the GM to scale back the power of enemies if they want their campaign to have a lot of longevity. This is exactly how most hardcore dungeon crawls worked in the old days - players were constantly encountering a handful of orcs, butchering them instantly, and moving on. There isn't anything necessarily wrong with fights being really easy, but it does mean that players are going to experience an awful lot of seemingly trivial encounters before something really bad happens and they are seriously worried about dying.
That situation of general easiness is especially difficult with big serious encounters against bosses and such. With really swingy combat you have to accept that the players are going to die a lot, or that they risk blowing up the boss on the first round and being really disappointed. It is much harder to achieve an epic feel when blowouts like that occur.
I think some people really don't mind that style. They feel okay if they end most fights having taken no damage and having just mashed their opponents. Some people really do like just waiting until the dice break against them to find their excitement.
Heck, some people enjoy every encounter being a near death experience and constant player deaths and party wipes.
Neither is really my thing. I like battles where the key turning points are decisions moreso than luck. I would prefer that the outcome is far more dependent on which abilities I use and which the enemies use than which of us rolls 20 on two d20s and explodes someone in a shower of gore. I like fights that use my resources and make me feel like I had to work for it. That doesn't mean every fight has to be 'balanced' but it does mean that the ones that feel tight and challenging really get me going.
I just don't get much value out of boring decisions creating huge effects based on the dice breaking weirdly. For me, that means that critical hits and fumbles are mostly a negative experience because they either wipe out the party, make potentially interesting fights into trivial affairs, or mean that the GM only serves up easy stuff and I feel more like a exterminator than a hero.