To be honest, anything will come with the risk of side effects. I went with the 10-week jab first, and ended up with eleven weeks of heavy bleeding - which along with a young child zapped my energy completely - and ended up having to go on hormone tablets to stop the flow. You'll find similar horror stories about the implant, or the pill, or anything really: anything that messes with your body chemistry comes with a chance of not being suitable, and the only way to find out is to try it.
I had my coil fitted in April, and love it. I used to suffer from insanely heavy and irregular periods, and now I get maybe a day of spotting once a month right on schedule, not even a full period, so for that reason alone, I'm over the moon. I alos had the vanishiong strings issue, can't find them at all myself, but had the clinic nurse have a look and it's all in order, just they've softened and shortened to such a point that they're rather well hidden (but the nurse did say if I wanted I could make an appointment at any time to have someone with a better viewpoint check for me, and told me about what signs should point to things being irregular, like pains or so forth, so I'm fairly comfortable with not being able to check myself).
Some women have reported issues like irregular bleeding or weight gain or headaches or whatever, but that's something you don't know until you try it, and if there's anything you're concerned about, it shouldn't be a problem to pop along and get checked out, and either reassured that it'll settle down or get it removed. That's actually why I opted to try the coil first over the implant, because if I ever needed it removed I can pop up to the gyno at the local health centre instead of having to make an appointment at the actual clinic. It's pretty simple to remove, barring any serious complications.
The insertion process is not pleasant, with sounds and so forth, but for me at least it was really just uncomfrotable and a day of mild cramping - less painful than period cramps - so nothing a couple of asprin and a bit of rest shouldn't be able to handle. for the first few months, periods did still come as heavy as before, although much more regular, with a fair amoutn of spotting and discharge, but that's normal, and worth a couple of moths of slight annoyance for a few years of not having to worry about contraception or possibly even periods altogether.
I'd say definitely go down to the clinc and talk to them about it. They're more than happy to give you all the information you need and talk through any concerns you have, and discuss what other options she might prefer if she's still wary, and let you go away to think about it, although I got mine fitted the day I went.
Just remember that it doesn't take effect contraception-wise instantly, so you'll still need an overlap time where you use both, but it is a long-term gain if it works for her, so a bit of patience is worth it.