Tfw you start playing again after more than a year and NU got Slowbro????
Anyway, first things first, this 'mon is busted. I was REALLY against the idea of banning pig until I sent Slowbro against it and got a z-wcharge on the face. That said, this is the problem number 1 with him: the versatility. You want to lure stuff or have a wincon against fat, slow teams? BU with z-move. You want to punch holes? CB. Your team is weak against the whole damn tier and you want to pretend it isn't? Just run scarf and outspeed everything while still hitting like a truck because why not. Having this many GOOD sets is probably what makes Emboar so broken. Those aren't some niche options that your grandfather used once in a civil war in Congo or something, they are straight up good to the point preparing (and playing) against him turns into a nightmare because there's no telling what the heck he will pull off. Take SPL and ladder as an example: in both, pretty much all Emboar sets, from scarf to z-move, are seeing usage AND success. The first has amazing coverage in just three moves, leaving the 4th to whatever else you need (Toxic, Head Smash, heck I've even used Sunny Day with Chlorophyll Venusaur); and the latter option pretty much steamrolls through slower teams by taking on things like Slowbro/king, Gastrodon, Druddigon, Pyukumuku, among others depending on what z-stone you are running. And that's not getting in details with other sets, like EBelt (less effective without Seismitoad around, but still usable) or CB, if you hate stall as much as I do.
Second of all, the tier shifts. With the drops of Venusaur and Heliolisk offering more options against water-types coupled with the rise of Seismitoad (which, in turn, made Klinklang, Toxicroak and Sneasel even better. Guess who checks those? Good boy), Emboar benefited SO DAMN MUCH. With venu and helio being great options in the tier, water-types are finding a harder time to thrive in this meta. Samurott, a viable check for Emboar on offensive teams, had his viability pretty much killed; Slowbro/king can no longer just be thrown mindlessly against the fire pig (not that they could before, because you know, the whole unimportant thing about it being more unpredictable than that xxpikachutrainer2006xx guy you face in low ladder) at the risk of giving a free turn to venu or helio; Silvally water proved to be disappointing to say the least compared to steel; and not a water-type but Druddigon also took a bit of a hit to his usage after losing his main selling point of SRing vs Xatu due to the drop in usage of the latter. And, as mentioned before, not only Emboar checks became less common and/or effective, but the things he checks are actually increasing in usage. Klinklang, Toxicroak, Sneasel, and the newly added Venusaur and Heliolisk are being used everywhere, meaning that 1) teams are becoming weaker to emboar, and 2) there's even less reason NOT to use it.
Third, one point that I think wasn't much (if at all) addressed is how bad other scarfers (or faster options) are in general, which only makes it even more of a pain for offense to deal with emboar. Now that we have Heliolisk, it became mandatory to run a ground-type in every single team, making scarf Rot-N way less common. Meanwhile, the increased usage of Slowbro means that Scarf Delphox also doesn't exactly have a nice time around (if it tricks the bro, then it loses to... Emboar). This pretty much leaves only Scyther, Houndoom, Vanilluxe and Braviary if Scarf, and Accelgor as faster options. You won't believe me if I tell you who beats 3/5 of those!
Last, how much it restricts teambuilding. I pretty much covered this point somewhat in all three other paragraphs, but to talk more about it, I must say that Emboar is unhealthy to the development of the tier itself. It restricts teambuilding to a fearsome degree. As of now, Slowbro already has more usage in SPL than the entirety of Snake Draft. Is that really a sign of a healthy tier? The stress it puts in the building process makes it so that there's little to no space for creativity when it comes to checks, or even to consider other options. Why run Scarf Delphox, if I can use Scarf Emboar? Why use Magmortar or Houndoom if you can use Z-Emboar? Why even use another fire-type at all? It makes teambuilding both lazy, because all teams will have Emboar doing whatever the hell they want to, and extremely stressful because, well, Emboar can do whatever the hell he wants to and there's little to be done about it.
In conclusion, the sheer versatility, coupled with more than favorable tier shifts and almost absolute lack of revenge killers (for the scarf set) makes Emboar a nightmare to play against and a toxic (for Slowbro!) presence in the tier, greatly hitting team diversity and making NU in general less fun to play. So, if it wasn't obvious already, it deserves a ban, preferably taking Heliolisk with him too.