Really? I tried using Clef as a cleric and wasn't really happy with it... And I'm probably the biggest Clefable fanboy in this thread. Its only edge over Sylveon imo are its abilities, maybe Moonlight if you don't plan to run Wish/Protect to free up a moveslot, and maaaaaaybe its superior movepool if you need one specific move to hit one specific poke. But overall I think Sylveon is the superior cleric. It doesn't completely outclass Clefable, but I think it's better in most cases .
Yeah, the abilities are kind of the point. Unaware and Magic Guard are both fantastic defensive abilities, and I personally used Clefable for Unaware, something that Sylveon could never do. I mean, I could see why you might think that Sylveon is better than Clefable, but calling Clefable garbage (I'm not saying that you have done this, but others have) is a step too far.
Sylveon's 110/65/130 defenses unquestionably make it a better cleric, so it is in no way "outclassed" by Clefable in that role.
Sylveon has a base 95 HP stat, not 110.
Well if its only about Hell Bell I guess Clef can get it done properly as well, its more about the wish passing part where Clef gets the short stick.
Well, when I say "cleric," I'm generally talking about Heal Bell. Whether it has Wish or not is another story.
The problem is simply that Clefables bulk is mediocre at best, even with full investment, and its offensive presence without boosts and LO is sub par as well. That means that it usually takes quite alot of dmg when coming in (you will have a hard time finding a physical attacker that cant deal 40%+ dmg to physical defensive clefable, LO/CB boosted attackers usually 2hko and with Unaware and rocks on the field basicly every physical attacker threatens to 2hko as well as most special attackers) and once it got in its hard for Clef to get a Wish of to heal something else. It doesnt force switches as much as Sylveon does so it generates less free turns and due to its mediocre bulk it cant just come in, take a hit, use wish and switch out as easy as Sylveon can.
The thing is, that's kinda debatable. Keep in mind that Sylveon has less physical bulk than Clefable does, and while it's special bulk is better, it's not like it has a field day with most special attackers like Chansey does. Assuming a physically defensive spread, it's in danger of a 2HKO from LO Latios's Psyshock after Stealth Rock, guaranteed with a little prior damage. You beat Latias, though. Aegislash smashes with Iron Head or Flash Cannon while Sylveon does little back. Charizard Y still 2HKOs with Fire Blast. You can beat Thundurus, but if it has Nasty Plot, it sets up and 2HKOs while you do 65% max in return. Unaware Clefable doesn't have that problem. Heatran is Heatran. Keldeo's LO Hydro Pump has a shot at 2HKOing, guaranteed with Stealth Rock or Specs > LO. You can beat Greninja from full health, but like Latios, you still take a ton of damage and have a small shot at being 2HKOed even after Stealth Rock. 0 SpA Mega Venusaur's Sludge Bomb is a 2HKO after Stealth Rock, and even without, it can just spam Sludge Bomb until you get poisoned and then 2HKO that way. You do beat LO Deoxys-S. Landorus's Earth Power has a small chance to 2HKO with no prior damage, nearly guaranteed with a Modest nature or Stealth Rock. Sludge Wave variants outright win if you switch directly into an Earth Power or Psychic or something. Gengar's Sludge Bomb 2HKOs, and LO variants have a shot at OHKOing after Stealth Rock. Mega Gardevoir's Hyper Voice is a nearly guaranteed 2HKO after Stealth Rock. You beat Mega Manectric pretty handily. Mega Alakazam also has a decent chance to 2HKO with Psyshock after Stealth Rock. Survivable, but too close for comfort. Manaphy 2HKOs with Surf at +3, while Unaware Clefable can actually stop it. Volcarona just sets up all over you.
And I guess that's about it for common special attackers. That also isn't taking into account the fact that Clefable's greater physical bulk and access to Unaware (to stop setup sweepers that would blow past Sylveon) or Magic Guard (to completely ignore damage from hazards, poison, burns, etc.) give it opportunities to come in safely and use Heal Bell / Wish that Sylveon would not have. Don't get me wrong, I realize that Clefable's unimpressive bulk is a problem, and it does let it down from time to time. But seeing as how Sylveon's main defensive perk is its much higher special bulk, yet it still struggles to tank hits from most of the metagame's top special attackers (especially tanking them comfortably enough to be able to use Wish for a teammate and not have to worry about healing itself), that advantage means a lot less than you might think it would. Besides that, Sylveon's other advantage is more damage, but it's not like it's wallbreaking any time soon. Clefable can also solve its issues with special power and bulk with CM + Bell, which is obviously a legitimate set, but that's probably irrelevant since I think you're referring more to WishTect + Bell.
Long story short, while Sylveon might not be completely outclassed, I do think that Clefable does give it legitimate competition as a cleric due to its better physical bulk and great defensive abilities (and this is also completely ignoring the existence of Chansey, by the way). Meanwhile, Clefable obviously takes the lead with its much wider versatility. I agree with New World Order that Baton Pass is a very real factor influencing Sylveon's viability right now, and based on the moveset stats, it's a big reason why it's being used as much as it is.