I played on Ridiculous/Ridiculous and high monster density on a huge map with 9 opponents lately. Overall I found it a lot easier than expected and I think one reason for this was the monster density. I don't know if this has been specificly pointed out by a post already. I only read about this in posts on other topics.

The huge number of high level monsters is a big problem for the AI. It is not able to handle them properly. The monsters regularly raze AI cities. I haven't found so many razed cities in and Expert/Expert game with normal monster density. If this could be improved I think the game would be more interesting and easier to balance. Currently I really don't know if the game would be more difficult if I played with insane Monsters.

One way to improve this would be to teach the AI to place its outpost better. I regularly see the AI place Outposts on monster lairs without the need. For example there was a Assassin Demon's Army and a Banshee and the AI placed its Outpost so that both started running around. It was absolutely not necessary for getting the resource. If the AI had avoided the lair it would even have gotten a second resource. Things like that happened all the time. It really doesn't seem to care if it disturbs a Darkling Camp or a Dragon.

If the AIs collected the loot from abandoned lairs in its territory it would prevent respawning of monsters. I think this would also help the AI greatly. If two spiders or a smaller Troll start running around AI territory it is not that bad, but the stronger monsters that spawn from the lairs later are really bad for the AI. There was a spiders lair near one of my cities in AI territory and I could observer how Magnar handled it. First the two widows started running around. The AI could have collected the loot in the lair, a Hero several times passed the lair but ignored it. Later a large Spider Army came out of the lair. The spiders killed several AI units before I killed them.