Pushing it counterproductive?

If I push a set of planche for instance so that the last couple of seconds the hips are getting lower than shoulders due to me getting tired, is that good / doesn't matter / actually harming progress?

yes it is counterproductive.Instead of holding for instance 1 minute planche and getting muscle failure,you can do a couple of sets of 20 and then do a couple of sets of 20 on the next day too.However if you are going for muscle mass building,then you should do the last set till failure.