Nickel is about 16 times more abundant than cobalt. I don't know if this matters, but cobalt is very toxic, much more so than even nickel. Cobalt is harder to fine. The aborigin tribes in Guatemala are suing their government for using their land to mind for cobalt, notably to be used in fuel batteries.
Cobalt, being an odd-numbered proton count, is of course less common than its adjacent elements.
Be that as it may, both cobalt and nickel can be very toxic when they are in organic form. Neither holds a candle to hexavalent chromium, though, and in metal form, both cobalt and nickel are very hard to get into a biological process. It's the organic side you have to worry about. None of them, or iron (which is also toxic in organic form in excess) are things you want lots and lots of in organic form.
Ironically, copper is right up there, too, but we handle copper wire all the time. This is an interesting (but not entirely definitive) read:
The toxicity of mercury (HgCl2), copper (CuCl2: 5 H2O), nickel (NiSO4: 6 H2O), lead (Pb(CH3COO)2: 3 H2O) and cobalt (CoCl2: 6 H2O) was studied under standardized conditions in embryos and larvae of the zebrafish,Brachydanio rerio. Exposures were started at the blastula stage (2–4 h after...
link.springer.com
The results are presented in the abstract, fortunately, thanks to the runaway monetization of all scientific publications.