@mzokk Thanks for your comprehensive response. You clearly have a grasp of "in the weeds internals" that I don't possess. I'm more of a "test and observe" kind of route planner. I look for instances of repeated routing behavior (both success and failures) across different terrain scenarios. I have multiple IBA (Iron Butt Association) awards where routing precision and mapping accuracy couldn't be compromised. I also lead multi-day rides across technical terrain in the US with very experienced riders who will not tolerate following "bad" routing. I bring this up not to beat my chest, but to indicate my required standards for building routes.
With respect, your response is filled with a lot of "ifs, and buts". I'm not challenging what you presented as anything other than real possibilities. What I am saying is that in my years of creating hundreds of routes using OSM for export to Garmin devices (XT, XT3) I have never encountered any of the behaviors you outlined. I wish I had so that I could pick your brain for further understanding.
I'm glad that you brought up the Zumo XT Forum. It's probably the best resource for learning everything XT out there. The main characters in the forum are incredibly knowledgeable. "jfheath" in particular has been very helpful in my XT learning journey. But even there, I sometimes scratch my head with some of the XT behaviors discussed. I have personally never experienced the infamous RUT behavior. That's over a 5-years period of continuous XT/XT3 route planning. Ironically, if you read through the forum, a couple of the regulars have sheepishly admitted they haven't experienced the behavior either. Again, this isn't questioning that it can happen, but it's clearly not a global pervasive problem.
Going back to HERE, I agree that in a perfect world that you would use the same maps for development that are used in execution. So within MRA, why not just use HERE?
If you didn't see my discussion on HERE map inaccuracies, start with the route below:
https://www.myrouteapp.com/route/open/14129162
Open this OSM route up, and compare with HERE. You'll quickly see that the HERE cartography has the road deviating in a completely wrong location. I can't imagine a more serious vendor map sin than not getting a road location right. Beyond this specific HERE snafu, there are several other MRA HERE flaws that I find unacceptable:
Too much of a lag time in recognizing closed roads that are now open.
Showing roads open that have been closed sometimes for years.
Calculating random route off/on deviations where the Freeway/Interstate continues straight, sometimes for hours.
I can go on, but I'll spare you.....
Given all this, for me, it's a matter of picking your poison. I can use HERE for development and know I'll get randomly tripped up, probably at the most inopportune time. Or instead use OSM, which has proven through extensive actual use to be reliable.
Just my thoughts on the matter.......