Abs can be sufficiently trained via isometric exercise. I believe the safest way to exercise the abdominals is to use them for their intended function (stabilization) under a progressively increasing load or difficulty.
For example:
The co-contraction of the abdominals with the spinal erectors that is required during heavy squats and deadlifts in order to stabilize the torso (http://startingstrength.com/articles/abs_rippetoe.pdf). As your squat and deadlift get heavier, your abs are forced to get stronger.
Stabilization during pull-ups or chin-ups. These will leave your abs sore.
In a rehabilitation setting, when trying to correct posture-related back-pain, isometric co-contraction is suggested over concentric abdominal work, as well as emphasizing the stabilizing role of the abdominals (http://portalsaudebrasil.com/artigospsb/holist006.pdf):
Exercise involving co-contraction of the abdominal and back muscles is also in line with stabilization.
A simultaneous isometric co-contraction of transversus [abdominal] and multifidus [back], while maintaining the spine in a static neutral position, should help re-educate the stabilizing role of these muscles.
Another publication supports the use of isometric abdominal exercises in a therapeutic setting (http://www.pitt.edu/~neurolab/publications/1997/BeimGM_1997_JSportRehab_Abdominal%20strengthening%20exercise-a%20comparative%20EMG%20study.pdf)
Isometric abdominal exercises [...] have also been found to improve low back pain.
