This can't be true. I was on the legacy build for a few weeks and still saw crashes. Mainly in Retroarch, NTRSelector, TWLoader, and a few games. (Mario Sports Superstars, Ridge Racer 3D, and a nifty indie I've been playing called Polara)
I'm more inclined to believe it has to do with game patching because even without Rosalina I still have the crashes. Enabling exception vectors only automatically reboots the 3DS, which is a bit quicker than having to push a button to shut down and then holding the power button until it boots back up but still not a fix.
I keep seeing "Use the legacy build" thrown around as a workaround and it doesn't fix anything for me. It simply isn't viable.
For what it's worth, B9S users can roll back to A9LH/Luma 7.1 IF you have a NAND backup from that setup. I do, so that's where I now sit. Severely outdated but stable.