For me, usually, the core functions have always worked just fine. The nice-to-have's are the ones that have occasionally taken the brunt when on betas - such as iCloud Tabs.
On the whole, being on iOS betas with the Mac system being release version is a more stable deal for me than being on both iOS and macOS betas at once. Wonder why.
I could not take Mojave betas on my 2016 MBP and iOS betas on my iPhone together. iCloud sync issues galore. Things worked with Mac on 10.13.6 whereas the iPhone on beta was no issue. Now, I am currently on a MacBook Air 2017 and gingerly running Catalina beta, while my phone is perennially on the latest beta available (currently iOS 13 beta 2). What I use my systems for is playing well together for the moment.