The number of pups in a natural and a side by side AI is about the same - there is not a noticeable decrease in fertility.
Of course you can put semen in a bitch via AI when she is not ready more easily than you can achieve a mating when she is not ready (she is more likely to permit a little tube going up there than a natural breeding when she is not ready) - so you may miss because, regardless of how you put it in, it has to go in at the right time. But between a natural and a side by side AI put in at the same time, it will have a very similar success rate.
Other types of AI, like TCI and surgical AI definitely see decreased success in terms of litter sizes and misses. Surgical AI has just been banned in the UK, without much in the news about this either. If surgical AI was banned in the US, there would be outcry(!) but since almost no one uses it in the UK at this point, there has been little to no reaction to this announcement by the BVA:
https://www.bva.co.uk/professional-development/vet-record/articles/canine-surgical-ai-now-a-prohibited-procedure/ My issues with AI are that: Dogs should be able to breed naturally. This is as important as conformation, as temperament, as performance qualities. If a male dog can't breed naturally, that is a huge fault in my eyes. (And if the breed cannot breed naturally, don't get me started!! It should not exist in its current form and breeders should be breeding away from any conformation which prevents natural matings!)
If the worry is about brucellosis (which we don't have in the UK), then I am a bit more sympathetic to AIs - although I wouldn't want to see them used in generation after generation - the real answer is to test male and female before breeding for brucellosis. There are not really any other STDs in dogs besides brucellosis to be concerned about and want to do an AI for.