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So I want to be very clear that, unlike Quilava in
Colosseum, I believe Jolteon is a still good Pokémon in
XD: Gale of Darkness. It can KO things consistently and is crazy fast. I just think it's a
tad overhyped.
Jolteon has two problems. The first is its fragility. This admittedly isn't an issue for most of the game due to how the difficulty curve works (Jolteon's bulk is actually decent compared to everything else in the beginning), but you start to notice how much damage it takes by the latter half of Cipher Key Lair and especially Citadark Isle. Glass cannons don't work too well in
XD's endgame unless they can hit a lot of Pokémon for super effective damage due to the doubles format, and Jolteon is a mono Electric with only Bite as coverage. It thus falls off kind of hard at the very end of the game because it can fail to OHKO things that aren't weak to its STAB and is in constant danger of fainting. You can use Thunder to close the power gap, but Jolteon either has to set Rain Dance itself, rely on a slower partner to do it, or raw dog those 70% accuracy checks at least once per fight and all options are awkward.
The second and more nitpicky issue is the potential need for a detour to grab Shock Wave's TM. Thundeshock's power starts noticeably wavering by the mid 20s (especially compared to some other options you have access to), so you kind of want that upgrade for the Snagem Hideout. This means doing the Mt. Battle fights to earn the TM and that's a decent waste of time. Then one dungeon after Snagem you go into Cipher Key Lair, where you can grab the Thunderbolt TM like. Immediately? The chest with the TM is in the basement, and the stairs to go down there are by the entrance. It just feels wrong to have to do the Mt. Battle rigmarole for a damage increase that gets mostly outdone soon afterward.
tl;dr still good for most of the game but falls off at the end, A rank mon instead of S