Planars use magnets on one or both sides of the diagphram. These magnets block the sound. The idea with their stealth magnets which are rounded on the outer edges is they are more acoustically transparent. Like Rayman30 says, they do tend to make a headphone brighter, particularly in the very high treble. It depends on other factors and the rest of the headphone as to whether this is a positive or not. The Sundara is less bright in the treble than most Hifimans, and many like it this way. The Sundara Closed Back, incidentally, uses stealth magnets and sounds much worse than the open back Sundara, I have both of them. But that's not from the stealth magnets.
Some (like Resolve) feel the Ananda Stealth was worse than the OG Ananda, particularly in the treble. I think I'd say it's better overall, but more as the bass is much better, I'd take the rest from the OG Ananda.
The issue with all this is it's hard to pin down exactly as there are few headphones where literally the only change is stealth magnets vs not, the Ananda Stealth while brighter in the high treble also improved the bass and that's unlikely to be anything to do with the stealth magnets. Sundara whether it's open back or closed is a much bigger difference than the magnets.
They brought them to the HE400SE well before the Sundara, I maybe have the feeling they realize adding them to that risks screwing up the tuning, which is somewhat unique in Hifiman, in that it has less recessed 1-2kHz and more restrained treble. That's what a lot of people like about it and it's not clear that stealth magnets would improve it.