What types of failure and how rare are they?

Lithium can cause kidney failure if not properly monitored and if folks don't heed the warnings of fluid intake, sodium monitoring, etc... that comes with the warning packaging

There isn't a single med out there for anything that will not cause some detrimental harm to some body organ over time, if not monitored, warnings and advisements not heeded, and well... taken.  Some folks are just set up with more sensitivity to chemicals and so the meds out there just cause all sorts of "other" medical problems and generally much worse than a lot of other people who take the same med(s).

There are also meds out there for psychiatric symptoms that actually cause psychiatric symptoms and/or cause "other" psychiatric symptoms to intensify.

There isn't a single med out there for anything that doesn't have some side effect and/or adverse reaction possibility and when you mix and match them... it tends to double the odds.

Still... most folks seemingly do well physically on meds and the symptoms they are taking them for tend to be relieved to a more tolerable level given time.  It's just everyone needs to be in contact with their docs, go to their lab appointments when scheduled, and heed the doc's advise if anything happens or shows up.