Cordless tools have come a long way. I have a cordless sawzall and there are other powerful tools that are now cordless, and they have the same power as a regular one.
I would be hesitant because weed-wacking is a more constant "on" activity usually, and I would be afraid that it wouldn't last long enough to get the job done.
The cord for the electric one, with a heavy long cord, didn't bother me that much.
On the other hand, one of my brother works in assembly and uses cordless tools, and he's tried to convince me they're much better now! I may try to find some user reviews on that model, but if it sounds like it won't do the job, I don't wanna waste my money. I'm talking about a tiny yard, so I would THINK it would be enough, but I don't want to risk it if it's likely to have less and less power time as it gets older.