There is a difference between default config and recommended.
As an example, the BTT based board example configurations have a BUFSIZE of 32. It is set that high for a reason. With any decent printing speed and the default Cura resolution (0.05mm), 4 commands can equal a whole 0.2mm. There quickly becomes a point in curves when even 115200 baud can't keep up with tiny steps - hence you have to have pre-buffered commands if printing via USB. This can even be an issue on SD card printing - let alone any other issues caused by factoring in a Pi etc.
ARC support is required to fix this problem - but I'm not aware of any common slicers that actually support it.
If you do chose to base everything on the default config, you should realise that the defaults are constrained to run on 8-bit boards with much less RAM than the boards you are building.
Yes, you did do work to show issues - however in most situations, your findings are tainted by badly tuned firmware parameters that drastically amplify the problems you were seeing.
Whichever board you are working on, if its an STM32 based, you should easily get a BUFSIZE of 64 to even allow tight and smooth curves at higher print speeds - and this even works with Pi's older / lower spec'ed than a Pi 3B+.
To assist with your research, I'd suggest enabling ADVANCED_OK which will show you the Planner Buffer (P) and Serial Buffer (B) available space to find other issues. Ideally, P should be zero, and B should be your max-1. That can easily be achieved on even 8bit MCUs.
Yes, I'm not surprised that you saw issues with the settings you used, but yes you can mitigate the majority of these issues with correct configuration in how you build Marlin.