I’ve participated in Battlecode for several years now, and have attempted a few CodinGame contests. But one issue I always have with the contests is I will make a decent bot, but as soon as I move into a new league a new rule completely changes effective strategies, or I have to rewrite lots of code to deal with new inputs. For example, in Coders Strike Back, you originally start with only one bot. But then, you later get two bots! When this new rule came up, I realized I would have to completely restructure my code, and just gave up.
I think the leagues are set up this way to make it easier to get started on a bot. But it seems to make it harder to continue working on a bot.