I’m not sure about Graphene specifically, but as long as there’s an established procedure documented, I’ve done like fifty installs of custom Recovery and various ROMs, such as Lineage, crDroid, Resurrection, Havoc, Cyanogen, AOSP, AOKP, ASPCA [:-)], WKRP [:-)], etc. and dozens of other less well-known ones, and I have not bricked one yet. So, as long as you don’t do anything blatantly foolish, should be ok.

I'd never heard of it being brick-prone. I suppose it's possible in the sense that updating a PC's BIOS could brick it if it loses power during the update, but I've never heard of Graphene's install process being particularly brick-prone.

I've personally flashed 4 pixels (friend's and family's as well as mine), a couple of them multiple times (I tried CalyxOS before Graphene with my first pixels), and it's always been a super smooth process, never had anything go wrong.