The common thread about those non-spam transactional e-mails you mention is also that they're usually barely above plaintext. They usually contain some minimally styled table, and some of the company colors. Maybe their logo somewhere. That's in stark contrast from the
spam these same companies send, which is where all that "Real Money To Spend on a graphic designer" goes. Compare e.g. the spam PayPal sends you vs. the e-mails confirming payments you've made. In my experience, this is pretty universal.
And speaking of universal heuristic, in my experience, the quality and relevance of content is strongly inversely correlated with the quality of design. The prettiest websites out there are ones that deliver negative value. The best designed (according to modern trends) user interfaces are the ones with worst ergonomy, wasting user time the most.
And yes, the subset of spam that's most recognized - ED pills, reproductive organ enlargement, members of royalty looking for help managing their finances - they tend to be very simply designed. But their distinguishing feature isn't simplicity of design. It's the carelessness. Typos, bad grammar, highly visible formatting mistakes, etc. When, on occasion, one of that "old school" spam messages tries to pose as a legit transactional e-mail, you can see through the deception by noticing the carelessness in replicating the design of the company being impersonated.