The Renesas RL-78 is one of the best MCUs in this price range (considering performance and power consumption), has fantastic free dev tools, and it's virtually unheard of in the U.S.
But if you do have a problem, good luck getting any support. Manufacturer will not respond unless you are buying millions, and there is no public support, since nobody uses them in hobby and small scale projects.
Okay, so Renesas won't help you, so how about others?
My experience with STM32 is that if you really need support, you won't get any.
Sure, if you are a total beginner and need to see a copypaste of an example code how to get a LED blinking, you'll get it from the forums. They are great for that.
With any real problem, you are on your own.
For example, when reporting a non-documented silicon bug and asking how to proceed from there, even as a business customer, ST just ignores you.
And ST is one of the most used and well known players on the ARM market, STM32 being a well known brand.
I guess it's the same game with everyone, really. We just have the feeling of "having support" when we work with the "well known" names, whereas in reality, we can only help ourselves. Starting from the risk that we never know if some part will be available tomorrow or not.
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Quality of documentation and the
exactness of it (i.e., is the product working within spec sheet / reference manual?) is, IMO, one of the most important criteria. Even a slightly poor choice of words can easily cost 10 hours of engineering time due to confusion and wrong assumptions. An omission of a small detail may cost another 10 hours. A copy-paste error or a typo is 10 hours more, then a undocumented silicon bug eats 50 hours extra. These do add up, this is typical design flow with STM32 in my experience.
Unfortunately, this is very time-consuming to evaluate. You'll know once you have fully stepped in and done full designs with said chips. And at that point, you consider yourself an experienced designer on said architecture, don't want to try anything else, since you already have "won" the battle and fear that another product family will bring you back to square one again.