Is there something I am wrong, or there really isn't a Granbull on the new tier list
Based on the Call to Action: Granbull article, it is slightly inferior to Gardevoir, so isn't it supposed to be in tier 5 along with other useful, but unfortunately outclassed Pokémon like Vaporeon, Houndoom, Shiftry, Absol, Sharpedo, and Poliwrath?
Answers
As far as I understand the Call to Action: Grandbull is setting this pokemon as a cheap option for a very specific niche. No stardust, no rare candy investment at all, but a decent counter for 2 upcoming tier 5 raids: Latios and Latias. That tiny niche is it´s only use, so it doesn't have room in a general tier list - as many other pokemon that are very useful against one unique boss, but irrelevant in the general meta otherwise.
GiardinoNI nailed it, but the key point is:
"No Stardust, no rare candy investment ...."
Much like finding a weather buffed Spheal at level 35 can give you a Walrein that is very effective against dragons for no dust or rare candy. It's a one trick pony that doesn't require expenditure of scarce resources.
