Unless we push past our comfortable limits in any endeavour there can be no progress. There are times when the body says "no more" when it attains a comfortable limit, but if given a little coaxing by the mind and others it can achieve more.
The key is to push past the comfort state in a gradual way so improvement is made, not to attempt a quantum leap to a place where training becomes dangerous as one is placed way outside their current capacity.
Safety should be the primary concern when pushing limits. One should be able to detect the difference between shaking up a person's comfort zone to bring about improvement, as against pushing the person into an area of true danger, beyond the limit of their maximum ability at the point in time.
I remember how challenging my very first grading in Aikido felt, at that point Dan anything was a delusion of grandeur... funny how now my dan gradings tend to have the same degree of relative difficulty that my kyu grades did at the time. Gradual pushing of the limits I guess.
Just my 2 cents.