I do, I see lots of issues with 6 months. you are taking formative hormones away from the dog when s/he needs it most. It affects personality, growth, and health. you have anecdotal evidence with your 3, but I also have it with my 5, my prev 2, and my sister's 2.
Mouse is not yet spayed (she's 3.5 years old but is allergic to metal and I am terrified that I'll lose her... she nearly lost her foot after we lanced an abscess with a stainless steel scalpel)
Scout was spayed at 4 (when I adopted her),
Zoe was spayed at 2.5 (when she trusted us enough to recover from the trauma of the vets we'd had her a year at that point),
Annabelle, my sister's Westie, was 14 months when she was spayed (3 months after her first heat),
Boo was castrated at 6.5 (when he was rescued from being a stud dog),
Riley, my sister's terrier x, we have no idea when he was castrated, (he came to the shelter already done and has some behavioural issues),
Benji, our little terrier x was somewhere between 1 and 3 when he was castrated (when we rescued him). He died if intestinal cancer 13 years later, but I attribute that to the Purina we fed the dogs... besides, he could very well have been 16 years old. he was perfectly healthy until he developed the cancer, and wasn't sick a day in his life and hadn't changed from the day we got him until he got sick. he was an extremely confident dog, who was well versed in dog language
Patches, my Springer x Cocker and Oliver, my Terrier x are/were the only dogs neutered at 6 months. Patchie went into heat young (5 months) and my mom didn't want to go through another one. She developed hypothyroidism (which has been proven to affect females spayed young at a higher rate than intact or spayed at or after maturity), had joint issues and was generally not a healthy dog. she also didn't tolerate other dogs much and was short and snippy with them. Oliver was the other, and he was only neutered at 6 months because the vet in Texas thought he was a year old, but he was just 6 months, so he got neutered accidentally young. He had major fear issues crop up that likely wouldn't have if he'd had the testosterone levels he should have had at 9 months old, he became afraid and aggressive with people, and he became very hard to handle. He's 4 now, and slowly getting better, but I really wish he hadn't been neutered! There are alot of studies out there to support this theory. Patches died at 14, very sick, hardly able to walk, deaf, blind, and with some unknown cancer (we weren't going to do diagnostics on her at that point)
as to female spay, the optimum time for the health and wellness for the dog is between the 2nd and 3rd heat. Now, this does not take into consideration irresponsible people that will let their dog get pregnant, which is by far a higher health risk than early spay
I will never neuter another dog early, I've had nothing but problems with the 2 neutered early, but the other 6 that weren't are/were perfectly healthy, normal, well adjusted dogs! The one we dont know about has behavioural problems and is dog aggressive, but we cant attribute it to anything because we have no idea when he was castrated, and no idea what his first 3 years were like.
BTW, Neutered is the umbrella term for Spayed and Castrated. It's technically for both male and female, but for some reason people normally use the term for male only. Possibly because it doesn't sound as bad as castrated LOL