Metallics don't work with a roller. The metallic elements need to be evenly distributed throughout the coat for it to look right. With a roller, the particles will inevitably end up at the edges of each roller stroke, leaving a stripey effect. trying to even them out will likely make it worse as you'll end up overworking the paint. It gets even worse on vertical panels, as gravity drags the paricles down.
When they're sprayed, the paint is dry enough when it hits the panel to prevent them flowing, and to keep them in place.

Even when sprayed, it's easy to get a tiger-stripe effect with metallics, with the edges of each spray stroke becoming visible if the strokes aren't just the right weight.

Metalflakes may act differently - as the flakes are larger than regular metallics/pearls, maybe they'll be less inclined to stripe, but I somehow doubt it.

With any flake job, you've got to put on a LOT of clearcoat to cover the flakes, and a LOT of wetsanding to flatten it all back again.

As with anything - maybe try a small test panel first and see how it works. Make sure you do it horizontally and vertically to get the different effects of gravity on the paint.