I think it entirely depends.
I have an acquaintance who is a SAHM, with husband. Her kid has no health issues, is in nursery 3 full days a week, grandparents have him every weekend, and also ad-hoc whenever she wants (feeling unwell, wants an extra day to see a friend etc).
Someone working, even without a child, has a much more stressful time than her.
But if someone was a SAHM, partner working, couldn't afford nursery, didn't have family support for childcare, and the kid had additional needs, then they certainly have a much more stressful and hard time than someone working a regular 9-5.
Unless that 9-5 person had their own health issues to deal with, or had kids with health issues and no family support of their own, etc.
My point is, it's impossible to compare the two unless we're comparing specific situations. There's too many variants.
The acquaintance I first spoke about LOVES saying how hard she has it being a SAHM. I never say it, but always think "would love to me able to send my job to my parents whenever I didn't feel like dealing with it" like she does her kid. Or to have someone else do my job 3 days a week or something.