There's a lot of SOS-chaining pokemon with quirks that you need to be aware of.
I was chaining Murkrows (breeding partner for a Mandibuzz + Foul Play), and much to my annoyance, Murkrows know Mean Look and Taunt. Over the SOS chain, they will use those two attacks. A Shell-bell on your Smeargle and Designated Chainer would help (or maybe U-Turn or Baton Pass). Other issues include Pancham / Pangoro Circle Throw, Low-chain Pokemon like Scyther or Sharpedo (just... low SOS Rates suck), Ditto (just don't switch in Magnezone like a dumb dumb).
I definitely count the PP of the opposing pokemon, as well as the chain. Whenever the opponent has less than 10 turns left, I KO it and start chaining off a new Pokemon. Beldum's 15PP Takedown was a special case however (Uggghhhh). I suggest that anybody who wants to chain a Beldum for a Metagross get a designated Alolan-Marowak / False Swipe, and definitely keep track of the 15 PP of the wild Pokemon.
Reminds me of a specific quirk to keep in mind while I was chaining Imposter Ditto. Somehow slipped my mind that Transform makes every move have 5 PP- which I didn't have a problem until about 40 minutes later when I forgot about keeping track and the Ditto struggled to death. (I usually swapped between Dittos but forgot after a while).
Speaking of, I also used Imposter Ditto to quickly check opposing Pokemon's ability and movesets if there is anything annoying to take note of (that was before I remembered about Entrainment). Keep in mind Imposter only copies the wild Pokemon on the left side of the battle and not the right.
Also Butterfree is a godsend throughout the game to catch and stop endless SOS chains. The set I use is:
Butterfree @none
Ability: Compoundeyes
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Sleep Powder
Stun Spore
Air Slash/U-turn
Thief
Thief+Compoundeyes is great for farming items like guaranteed Leftovers from Munchlax (especially when paired with Thief Weavile). Double Powder for versatility depending on mon (Stun Spore for a permanent stop to chains, Sleep Powder for Electrics). Air Slash is generally useful for the Grass types you want to chain and also was VERY useful trying to find Razor Claws in Jangmo-o chains. U-turn for momentum to defeat a low HP partner pokemon as you switch to your False Swiper.
Also if you are the type to look for specific-coloured Miniors, the trade Poliwhirl named 'Whirly' is pretty useful straight away. Use a Water Stone immediately fresh from the trade to get a Poliwrath with Damp. Waterium-Z Bubble does *just* enough damage to Minior without KO-ing them, enough to reveal what colour the core is. Damp cancels out Self-destructs while Fighting means you resist the suddenly stronger Power Gems. If you don't like the colour of the Minior- simply run away since levelling up Whirly means more of chance of KOing the Miniors (and because it was traded, it levels up pretty fast).
'Whirly' the
Poliwrath (aka Minior-Miner) @Waterium-Z
Ability: Damp
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-Bubble
-Hypnosis (optional, needs Heart Scale)
-Any
-Any
I find Whirly was better than Parasect during my second in-game playthrough since Parasect is weak to Power Gem + False Swipe isn't as good against Rock typing while Meteor form Minior is immune to Spore. Hypnosis is optional if you ever want to up the catch rate but unfortunately Whirly has just forgotten that move so it's up to you if you want to relearn it. Didn't feel the need to since I usually caught the Miniors with no problem.