Yep. This is great until you need a cross-browser date picker, at which point you need to implement a bunch of stuff yourself. It’s frustrating how primitive HTML forms are, after so many years.
What's hilarious is they do have UI for it in about:config "dom.forms.datetime.timepicker". It makes me so angry that it's not on by default. It works fine!
That you’re correctly using html forms won’t quickly lead to browser improvements.. so the result is that users will hate your forms.
Users/your customer might possibly even think that you’re to blame, and not $browserVendor.
I’ll go one further and say that the customers are absolutely justified to blame the developer instead of the browser. If a developer knowingly chooses a built-in form control whose common implementations are bad for their users, how are they not at fault for the resulting experience?
“This site only uses functionality provided by the HTML spec” is not a useful goal in and of itself. Using the right tool for the job, which might be JavaScript, is always more important.