I've had Mirena since 2011 and it's great. Any side effects I've had have been minor and not bothersome enough for me to worry about. I got it for helping with heavy bleeding and it has been literally the only thing that has helped with that.
You're going to find a lot more negatives about any method of bc online because people only go online to complain about stuff and to see if other people have had similar problems. So you're only getting the negative side. There are literally tons and tons of women out there who have gotten either Mirena or Nexplanon and have had little to no problems so they are not online posting their experiences because they are out there living their lives. You need to realize that you'll never get the true picture of actual positive experiences because those people rarely if ever post and there are a lot more of them than there are the negative stories.
So my suggestion is that you try for yourself and see if you like it. Neither option is permanent and you can get them removed at any time. It's not that big of a deal. I was afraid of mirena before I got it because of the horror stories. Once I got it and saw how it was for me, I felt pretty stupid for putting it off for so long because like I said before it's the only thing that has worked for my heavy bleeding. I could've had relief a lot sooner if I wasn't so afraid of it before. Now I'm like, let's get the new one as soon as this one is expired!
I'm not to hip on having something inserted in my vagina
That's my only problem with birth control is that it makes me bleed like crazy. Wearing pads and tampons all year is gets expensive and what's the point of being on birth control if you can't have sex dude to the birth control. I remember one doctor told me to just take 800 mg of ibuprofen three times a day to stop the bleeding which it worked but it's dangerous to continuously take pain medication like that and that's when i was on Implanon
I'm seriously considering the Mirena IUD. I can take the pill but it's a very particular kind - I was on two in my life then they stopped producing the first, and the second they put me on made me tired all of the time and I bled like crazy.