with the price of dremel bits and how many you need to cut metal, id consider a small angle grinder and some 1mm discs , halfods had grinders on offer for £15 plus say £1 for a disc that will laugh at nuts n bolts
The slitting discs on dremels are very fragile, it would be my last choice for a stuck fastner and i've only every used it if I really can't get anything else in there. If you're talking nuts then nut splitters are cheap and effective, but don't really work for bolts
I use them all the time and yes they are fragile if you put a side loading on them but if used carefully they are great. There is a mandrel you need to use them and they are a click and turn jobbie so fitting and removing the discs takes seconds. You buy the discs in packs spring loaded so they slip out of the carrier easy. Try ebay as that is where I get mine. You can also get grinding discs to fit too!