how safe are wheel adapters???

The billet kind are more than strong/safe enough. Plenty of guys use them for autocross as well. My g/f ran a set on her truck for years with no issues, and as others have posted the rock crawler types use them too.

I'd be inclined to go the disk/axles route myself, but not because the spacer/adapters aren't safe. Its just the better way to go.

Besides, you should be able to get 15x7 rallye's in a 5x4" pattern anyway, if that's the only reason for the bolt pattern swap.