IUDs can cause scarring, cramping and heavy cycles. Not all the time and not for everyone. They are long term and so can be convenient. But not recommended for women who have not yet had a child. I guess if you've not had a child you may be more prone to the type of scarring that could prevent pregnancy.
The link between IUD's and infertility isn't a very strong one. IUD's themselves don't cause scarring. However, can increase your risk of developing Pelvic Inflammatory Disease (PID), which is a complication of certain Sexually Transmitted Infections like Gonorrhea and Chlamydia. Basically, if you have an STI when the IUD is placed, it can carry the infection to your fallopian tubes and ovaries, resulting in PID. Its the PID, in turn, that can cause significant scarring leading to infertility. If you're in a mutually monogamous relationship, the risk of STI's is minimal, which means you shouldn't have any problems with PID or infertility with an IUD. As for bleeding, non-hormonal IUD's do cause heavier periods in a lot of patients. Hormonal IUDs however, can cause spotting for the first few months, but afterwards, many patients don't have a period at all for as long as they keep the IUD.
Ultimately, this is probably one of the easiest, most effective, reversible forms of contraception available (less than 1% failure per year).
But if they are correctly placed, they are very effective and have the benefit of not ingesting hormones.
Diaphragms actually aren't much more effective than condoms with "typical" use (failure rate of 15-20% per year).