I used either la bella jamerson flats or fender nylon wounds depending on the tension/tuning I'm using (la bella > fender in terms of tension). Never had one break unless I am going heavier with a BEAD tuning and don't file the nut enough. More often than not with my experience with flats, they tend not to fail on their own, it is usually something with setup like others have mentioned. IE burrs, nuts too narrow, etc.