I am now using Acrylic Paint Pens for signatures. Some of the card surfaces are such they repel Sharpies, leading to a streaky or beaded appearance, and in some instances they barely show up at all. This has always been a thing but it seems to be happening more of late, even on older cards, leading me to think that it's Sharpie production quality that's at fault.
The advantage of Acrylic Paint Pens is that they allow for a more solid, defined line, and colours are more striking. The flipside is that Acrylic is not as durable as Sharpie ink, meaning they need to be protected from excessive friction and scratching, although that tends not to be an issue with cards as of course you treat them so well.
As far as the signage goes, Acrylic Pen paint takes an age to dry, as opposed to the Sharpies which are instant. So where with Sharpies it's two outlines and into the sleeve, with Acrylic it's one outline, wait an hour, second outline, wait an hour and into the sleeve. Also, any overlap results in blotching, unlike Sharpies. For these reasons, my prices for Shadows have increased.