Pretty much every viable scarfer in the tier can OHKO Mega Pidgeot after rocks. Many have multiple move choices to do it:The worse part about M-Pidgeot is that it restricts teambuilding SO much, if you dont have one check or a revenge killer that can reliably kill it (only rotom-h and rotom-mow, or you may as well use jolteon) you may just lose to it at team preview sadly. I shouldnt be punished for not having a M-Aerodactyl just to deal with M-pidgeot, that isnt how it should work...
252 SpA Hydreigon Draco Meteor vs. 0 HP / 0 SpD Mega Pidgeot: 249-294 (81.1 - 95.7%) -- guaranteed OHKO after Stealth Rock
252 Atk Mienshao Stone Edge vs. 0 HP / 0 Def Mega Pidgeot: 256-302 (83.3 - 98.3%) -- guaranteed OHKO after Stealth Rock
252 Atk Mienshao High Jump Kick vs. 0 HP / 0 Def Mega Pidgeot: 249-294 (81.1 - 95.7%) -- guaranteed OHKO after Stealth Rock
252 Atk Sheer Force Darmanitan Flare Blitz vs. 0 HP / 0 Def Mega Pidgeot: 324-382 (105.5 - 124.4%) -- guaranteed OHKO
252 Atk Sheer Force Darmanitan Rock Slide vs. 0 HP / 0 Def Mega Pidgeot: 272-322 (88.5 - 104.8%) -- guaranteed OHKO after Stealth Rock
252+ Atk Salamence Outrage vs. 0 HP / 0 Def Mega Pidgeot: 267-315 (86.9 - 102.6%) -- guaranteed OHKO after Stealth Rock
252 Atk Heracross Stone Edge vs. 0 HP / 0 Def Mega Pidgeot: 256-302 (83.3 - 98.3%) -- guaranteed OHKO after Stealth Rock
252 Atk Krookodile Stone Edge vs. 0 HP / 0 Def Mega Pidgeot: 244-288 (79.4 - 93.8%) -- guaranteed OHKO after Stealth Rock
252 SpA Chandelure Fire Blast vs. 0 HP / 0 SpD Mega Pidgeot: 235-277 (76.5 - 90.2%) -- guaranteed 2HKO
252+ SpA Moltres Fire Blast vs. 0 HP / 0 SpD Mega Pidgeot: 231-273 (75.2 - 88.9%) -- guaranteed OHKO after Stealth Rock
252+ SpA Moltres Hurricane vs. 0 HP / 0 SpD Mega Pidgeot: 231-273 (75.2 - 88.9%) -- guaranteed OHKO after Stealth Rock
252 Atk Mienshao Stone Edge vs. 0 HP / 0 Def Mega Pidgeot: 256-302 (83.3 - 98.3%) -- guaranteed OHKO after Stealth Rock
252 Atk Mienshao High Jump Kick vs. 0 HP / 0 Def Mega Pidgeot: 249-294 (81.1 - 95.7%) -- guaranteed OHKO after Stealth Rock
252 Atk Sheer Force Darmanitan Flare Blitz vs. 0 HP / 0 Def Mega Pidgeot: 324-382 (105.5 - 124.4%) -- guaranteed OHKO
252 Atk Sheer Force Darmanitan Rock Slide vs. 0 HP / 0 Def Mega Pidgeot: 272-322 (88.5 - 104.8%) -- guaranteed OHKO after Stealth Rock
252+ Atk Salamence Outrage vs. 0 HP / 0 Def Mega Pidgeot: 267-315 (86.9 - 102.6%) -- guaranteed OHKO after Stealth Rock
252 Atk Heracross Stone Edge vs. 0 HP / 0 Def Mega Pidgeot: 256-302 (83.3 - 98.3%) -- guaranteed OHKO after Stealth Rock
252 Atk Krookodile Stone Edge vs. 0 HP / 0 Def Mega Pidgeot: 244-288 (79.4 - 93.8%) -- guaranteed OHKO after Stealth Rock
252 SpA Chandelure Fire Blast vs. 0 HP / 0 SpD Mega Pidgeot: 235-277 (76.5 - 90.2%) -- guaranteed 2HKO
252+ SpA Moltres Fire Blast vs. 0 HP / 0 SpD Mega Pidgeot: 231-273 (75.2 - 88.9%) -- guaranteed OHKO after Stealth Rock
252+ SpA Moltres Hurricane vs. 0 HP / 0 SpD Mega Pidgeot: 231-273 (75.2 - 88.9%) -- guaranteed OHKO after Stealth Rock
Maybe if the Agility set were more of a thing then Scarfers wouldn't be such a strong answer, but as it stands there are many options to revenge-kill Pidgeot. The Work Up set does give a middle finger to special walls, but even then Umbreon kills it with Foul Play and the set's lack of coverage means a Steel-type will stop it in its tracks. Mega Pidgeot is an awesome win condition that cleans up nicely once its checks are gone.
As for Feraligatr, I haven't found him to be a big issue yet. He's just got mediocre bulk and speed before setting up, and his lack of recovery means there are lots of ways to stop him. Swords Dance sets break walls, but get revenge-killed by anything moderately fast. Agility sets are great late-game cleaners, but a healthy wall stops them. Dragon Dance sets, I've found to be the worst of both worlds as a lot of stuff still outspeeds at +1 and a lot of stuff still avoids OHKO at +1. It's still a very dangerous pokemon, but most of the time my Scarf Hydreigon just comes out and stops its would-be sweep after it gets one kill.
Both of these are very powerful pokemon, but hardly unstoppable.