Dental implants aren't perfect
As with everything, nothing is perfect. Dental implants, when done right, are the longest lasting dental restoration.
Some problems would include:
Not getting the implant completely embedded in bone
Not getting the bite correct, thus losing supporting bone
Patient with history of bisphosphonates, thus no fusing of bone to implant
Incomplete removal of cement from the crown on top of the implant, thus losing supporting bone