Lemmiwinks MkII said:- Awful special defense, meaning that it can’t reliably take strong special Water attacks among other things. Also means that it is vulnerable to the odd special attack from Pokemon it is called upon to wall, such as Dragon Dance Lapras and Sharpedo
Can Slowking take those Pokemon? No. This would be a valid point otherwise.
Lemmiwinks MkII said:- Many strong Fire types (Flareon, Entei, Charizard, Camerupt and Rapidash to name a few) often go at least partly physical, and Leafeon has no chance of even sponging a hit from any of them. Slowking can take at least one super-effective hit from any of them, if not wall completely, and retaliate hard.
They can still reliably break Slowking, though. At least from my experience.
Lemmwinks MkII said:- Leafeon struggles badly against most weather barring Sand. Rain, Sun and Hail teams plough through it, whereas Slowking checks Rain and Hail quite well, and can at least contain the odd Pokemon in the sun.
In the sun, it can switch in on Sleep Powder, the odd SolarBeam (not from exeggutor, and not to mention stall with Protect.
Lemmwinks MkII said:- No access to Thunder Wave or Trick Room, making it hard to slow down fast offense. Defensive Leafeon has to invest mainly in defenses to perform its job, so its high base speed goes largely to waste against these threats.
It doesn't go to waste when considering you need to outpace Pokemon such as Rhydon so that you don't get raped by Megahorn (like Slowking does).
Lemmwinks MkII said:- No Fighting resist, which is the most important team cover that Munchlax needs. It’s not just Medicham, Banded Close Combats from the likes of Hitmonchan and Primeape can comfortably 2HKO Leafeon, but not Slowking. Even when not choiced, the latter can dispatch you promptly with Overheat.
It isn't a "reliable" enough Fighting check, in my opinion. I think you need a Ghost in NU for the sole purpose of reliably checking Fighting types. There are Pokemon such as Primape can completely plow through Slowking with U-turn, while Hitmonchan and Medicham can both still break through Slowking. If fighting-resists are that important, I would definitely not rely on something like Slowking, I would use a Ghost.
On top of this, Leafeon boasts many important resistances, namely to Ground and Grass. Munchlax + Slowking are absolutely destroyed by anything like Shiftry, Offensive Leafeon, Victreebell, etc because Slowking simply doesn't have the Defense to compensate for the weakness. Leafeon can at least tolerate shit like U-turn from Primape, and even like Fire Fangs from Flareon and Luxray.
Lemmiwinks MkII said:- No reliably strong attack to hit Pinsir, Skuntank and friends, common physical threats that Slowking can at least deter somewhat. Also loses to Muk and Qwilfish, two Pokemon that Slowking can beat easily.
Pinsir and Skuntank will absolutely obliterate Slowking. Much faster than Leafeon anyway, who can at least take a hit from each. Muk + Qwifish two exceptions to the "Leafeon is always better" rule, I guess.
While many of these points are at least semi-valid, they really don't outweigh the benefits of having legitimate checks physical attackers (their main job), at least in my opinion.