My kids will be about 14 months apart in age. I've had no major complications, but at the same time I wouldn't intentionally choose that small of gap. The biggest thing for me was my decreasing and eventually disappearing milk supply affecting my son's health in ways that proved rectifiable but nevertheless were risky -- doesn't happen to everyone, but then again it can't be predicted either. Also there is the simple level of exhaustion in trying to chase after a one year old while in the third trimester, the sense of not having really recovered from the level of attention/care/sleeplessness a newborn can demand before starting it all over again, and the degree of tiredness/soreness in all muscle groups related to supporting my unborn child before they had a chance to recover themselves -- all things that can come with a subsequent birth even years later, but which I feel are exacerbated by being so early. I am concerned, as well, as to what a second birth so soon might do to the scar tissue from where I tore with my first.
Obviously none of that is "oh my god the trauma!" kind of stuff, but all together it IS a greater degree of discomfort than I think I was prepared to take on. For some women, of course, one baby after another is great. But I think it's pretty normal to benefit both physically and emotionally from a little bit greater gap.