I am definitely not a fan of Convenia because there are way too many downsides associated with it. Some vets use it for everything

Number one reason I won’t allow it in my cats is the risk of hemolytic anemia. Also if your cat has a negative reaction to it there’s no way to get it out of the cat’s body. Some cats require hospitalization and supportive care to get through it. Having said that, I have had a kitty who received a Convenia injection and she was okay. The vet did it without my permission. I was ticked off! But that’s COVID veterinary medicine for you… I wasn’t allowed inside and the vet drained a massive abscess from this kitty and gave her the Convenia injection. At least that was more appropriate to the situation since Convenia is designed to treat skin infections - but I was still mad and will never take my cats back there anymore because this guy tries to give Convenia for everything- even flea allergy dermatitis!
I have also had to use it in feral cats before, but in that case pilling was not an option.