MoT's WDF

This is the type of thing I am referring to - https://www.ebay.com/itm/143300735480?c ... 2216108866

I find them to be super convenient and comfortable. Any downsides to using a magnetic gadget with a modern watch?

Well if you use it with a mechanical (spring driven) watch it could affect accuracy. Some premium brands design in magnetic protection.

https://wornandwound.com/watches-and-magnetization/

I'm not aware of any effects on quartz watches.