There's a reason why I don't want a string trimmer, nope, no way. We gave up on string trimmers years ago. The "string" is dangerous, and a pain to deal with - always getting caught or tangled. Injuries and backaches occurred.
The 2-cycle engine was also a pain - at least we don't need to deal with those any more.
The string in my old gas trimmer would sometimes jam, but I have not had a single issue with my EGO string trimmer. Yeah, you have to wind new string on when it's empty, but that's a simple job that only takes 3-4 minutes and you're off and cutting again. Unless you have a lot of cutting to do, you may only need to wind new string once or twice each summer.
I'm not sure what you mean by string being "dangerous"? While I wouldn't recommend trimming barefoot in your flip flops, I've accidentally hit my shoe a time or two with no issues. I wouldn't make a habit of it, but it's hardly dangerous. Of course, you should wear eye protection with any outdoor tool that flings things (edgers, trimmers, chainsaws, etc.).
My EGO battery powered trimmer actually has more power and a wider cutting path than my old gas trimmer. The only advantage my gas trimmer had was a brush cutting blade. I used that a lot when we first got our property to cut through brush and small trees up to an inch or more thick. But with a defined yard now, I haven't needed to do that in many years and can always grab my limb clipper if needed.
As for backaches, you can get shoulder straps for most trimmers (gas or battery). I bought one for my EGO but it's a little too short so I never use it. Thankfully, the EGO trimmer is lighter than my old gas trimmer. Adjusting the handle so the trimmer is balanced for you helps too.
As for edging a lawn, as long as it's already edged, the trimmer can easily maintain the edge. If you've let the grass grow out over a sidewalk or something, you might need to rent an edger or get an electric one to get things cleaned up.
I also use my string trimmer to clear the moss that grows on sidewalks or between pavers. It uses up the string a bit faster, but it beats kneeling down and cleaning it out by hand.