I have had spaniels my whole life. Owned, adopted and fostered.
I would say first off, you will get people who have got a cocker or sprocker as their first dog, with young kids, and it worked our wonderfully well. It does happen.
However, the vast majority we see through the rescue are first time owners with young kids that either don't have the time to put into enrichment, don't understand enrichment and how it can be over stimulating, can not seem to train the puppy that have bite inhibition, find them too full on or aren't prepared to deal with resource guarding and separation anxiety.
And yes, lots of cockers don't resource guard (non of mine have) lots don't have separation anxiety, one of my rescues does have this. But if you join Facebook pages like 'cocker spaniels UK' you will see the frequency that people are looking to rehome, due to these issues. Or posting because they are at the end of their teather with their behaviour.
The line between tiring the puppy out and over stimulating them, is often very fine.
As, pp says, they are known as cockerdiles as pups and many families find that the kids end up disliking the dog until its much older. Or the kids start screaming and the dog becomes over stimulated. People think they are prepared for the biting, but often they aren't. And it can take anything from weeks to months to sort.
I think they make fabulous family dogs, but they are a breed that you can easily get it wrong with. When it gone wrong it can be difficult to undo.
Our youngest spaniel, was one of 3 that I fostered at 5-6 weeks old after the 'breeder' threaten to dump them. She is an excellent companion for my ds who is autistic. They are inseparable when my ds is here. She knows when he is getting stressed usual at bedtime and she lays on him. The pressure helps him sleep then she moves off but stays near him.
My other, is the lesser spotted lazy show/working cross and is the eaisest dog I have ever had.
Their recall as always been fabulous, even in the teenage stage. I use total recall be pippa mattinson, which is an excellent book.
Also the sheer amount of cocker breeders at the moment is worrying. So many are ones that wi give you the 'we just bred our dog because we love them so much and wanted one like them'. Which is code for 'I am a backyard breeder and don't know what I am doing'.
There's also a huge amount of people paying for deposits and then not getting the dog. The breeder comes up with some random excuse and its sold to someone else. Usually, a higher bidder. These breeders also won't ensure you are fully prepared. Hence so many going through rescue.
Sorry for the long post, but the last year as been ridiculous for rehoming, mainly due to unprepared owners and breeders who are not responsibly breeding and it appears to only be getting worse because puppy prices are so high.
As for sex, I find it's more a personality trait rather than sex. Some females are more loyal, some males are. Some females like other dogs, some are more aggressive to other dogs. Same with males.
Oh, they also usually shed, often have anal gland problems, can have trouble with 'happy tail'
I feel like I have had a huge rant about how bad they are. They aren't, they are fabulous, but a breed I wouldnt recommend for a house with young children, where there very little experience with dogs.