No chance on bricking the host computer, unless you are really careless and manage to physically damage something in the process. On the 99/4A the original VDP is already in a socket, so the biggest challenges (as already mentioned) is disassembly of the console, and mounting the video connector. For other systems, like the CV or ADAM, the original VDP is soldered to the motherboard, so installing in those systems is more difficult since you have to desolder the VDP and install a socket.
Also note in some case the F18A can "unbrick" a system, since a common failure of the old computers is the DRAM chips that were used to implement the VRAM, and the F18A has its own on-board VRAM and does not use the original VRAM in the host system.
My apologizes for the delay in getting more F18A boards, I'm working on it as much as possible. It will happen, just not as quickly as everyone (including myself) would like.