very tough cuts of meat can be boiled, braised or simmered. but that's not the case for sausages.
i never boil sausage before cooking. all kinds of fresh sausage from italian pork to herb/poultry sausage is always grilled or browned on fairly high temperature.
on the grill, the trick is to get the inside to be just cooked thru as the casing browns and just chars a bit. if the fire is too low, it will overcook inside and dry out before the casing appears done. if the fire is too high, the casing will burn by the time the filling is cooked.
grilling is the only method for a sausages and peppers hero, imo. the sausages grilled, the onions/peppers/garlic/oregano/bay leaves/red wine cooked in a seperate pan, then the sandwich is assembled, with a little plain tomato sauce offered as a condiment.
if the sausages are bound for a pot of sauce or a beer/onion bath, then it's ok to fry them in a little oil (or grill them) to get good color on the casing, but not cook them thru. they will finish in the sauce or bath.
in nomine patri, et fili, et spiritus sancti.
Meh nom eh noh...doot dooooo do do do.