Slack action is (sometimes) used to get a heavy train rolling with insufficient tractive force. Of course, it is not good practice, and there is a high risk of train separation.

As my background is European, there are known cases where a train has been taken over by the Deutsche Bahn from the Swiss Federal Railways, and they had serious problems getting it rolling. The Swiss couple rather tight, and provide rather high tractive effort (because even a "flat" line may have 1.5% grades), whereas the Germans couple looser, to haul a train with less tractive effort. Note that power rating does not really matter that much; power rating determines at which speed the maximum tractive force no longer can be applied (because the engine "runs out of steam").

If you have a pusher engine, there is the advantage that you can use the dynamic brakes at maximum power. Again, an European example: there were quite heavy trains over the Lötschberg line (before the base tunnel opened), and they actually had more power at the rear than in front, because going down the 2.7% grades, the pushers could get to the maximum regenerative braking rating, meaning maximum braking force, whereas the leading engine had a reduced braking force because of the risks that cars may be pushed out of the consist in tight curves or switches. With that configuration, they controlled the train just with regenerative braking, and needed air only for the last moment when it had to stop.