I have a bow with a micarta riser. It’s a short riser, so I asked the bowyer to just use micarta to bump up the mass.
I don’t think there is any other performance aspect to it.
There are two types of micarta typically used in bows. Linen and paper. Bow tips are usually made with linen as it is stronger but lighter, and more expensive. Risers are usually made with paper as it is heavier and less expensive.
Bob Lee makes a lot of risers with it. Black Widow puts it in all their risers too. It’s a fine material to make a riser from.
I wouldn’t hesitate a bit about getting it. In fact, I would prefer that to tropical woods. I don’t use any tropical woods in the bows I make ’cause I don’t want to participate in any way in the deforestation going on. So I have a hard time getting the weight of my risers up. Micarta is a good way to get a heavy riser without the heavy ecological cost.