I have the DR walk-behind string trimmer, which I believe uses the same head and string as the fence trimmer. The heavy-duty string lasts quite a while, and does heavy grass, stemmed weeds, and even first-season re-growth of cut-off trees if they are still very green. You can break it, mainly by getting the mowing head right next to a rock or other hard object, so the string is hit near the base. I'd not think that's as much of a problem with the fence trimmer because it has the circular guard which steers it around posts.
I've not seen the reviews that claim it's hard to change the strings, but from my experience I think they are just being whiny. No, it's not as easy as just tapping the head to the ground like you do with light hand-held, but it's no big deal either. If the old string has been there a while you push it in a bit until it loosens, then pull one end to remove it. Then push the new one in. I use the smaller holes for the heavy-duty string, which makes them hold better, so you have to put a bit of muscle into it sometimes, but nothing my 73-year old hands can't deal with with no problem.
Terry