I have been playing Medmod 3.14 for quite a while and I have played most of the factions in Early and High starts. Medmod does 4 things that really change the game.

First, if causes the AI to actually successfully trade, maybe with a little too much emphasis on boat building but that may have been required to get them to build enough boats in the first place. It is not unusual to have the Danes with a full stack of 16 boats in a sea zone in the later parts of the game. This has caused the AI to have lots of money and has led to the second benefit.

The AI armies are usually as good as those you build. No longer will you face peasants and urban militia armies. Also, because the AI has money enough for troops there are many less rebellions, usually only being caused by unloyal princes or the death of the king without heirs.

Third, the AIs have been set to be aggressive. This is quite obvious when you are attacked by one faction and all the other ones see this as a sign of your weakness and pounce on you too. It is not uncommon to be attacked by 5 factions simultaneously. You WILL actually lose some games if you play Medmod. In particular, if you try your hand at an HRE game, you will lose a few times before you figure it out.

Fourth, you have been weakened somewhat although these features also affect the AI. You are only allowed to build troops in your HOME provinces. This means that acquiring these Home provinces early enough to build infrastructure is very important. It also means that you can't flood the board with troops as easily. It also means that the quality of your troops are more important.

There are many changes to the units in Medmod and they cause each faction to FEEL and PLAY very differently. Try it, if you are hankering for a challenge you will never go back.