I was actually happy when my DS was diagnosed with AS...but that's because of how long it took, he'd by that point been having SLT for 11 years, OT for 7 years and assessed every 6 months or so for 6 years, his school were adamant he was autistic, I was pretty sure he was and every 6 months the specialist said that while he was showing traits and met most of the diagnostic criteria because of his speech, she wasn't willing to diagnose yet.
So all that time, the little autism specific support there is wasn't accessible to us, he struggled more and more through primary school and into secondary school, without a diagnosis of anything secondary school withdrew support until he had a complete meltdown and then they suddenly realized he wasn't coping and had to refer him to the mental health team (they're separate from autism diagnosis in my area), his self esteem was so low that expressing suicidal thoughts wasn't uncommon - because he only deals in absolutes, so you might have autism just wasn't relevant to him.
So to finally get a diagnosis of anything was just fantastic, he could go to a youth club for children with autism, we could actually properly discuss the fact that he wasn't just rubbish at things and unlikeable that it was that he was wired differently and there are other people in the same boat, I finally had something that would make his school take his difficulties seriously and access to things like the autism outreach team in school...
I understand that for someone who has a quick diagnostic process that it will be upsetting to have somebody officially tell you that your child has a disability, but for us the diagnosis wasn't devastating, the period where he was undiagnosed and completely floundering was.