Sorry, what I meant was not an overall difficulty increase but that when you unlock more weapons, the game will spawn harder enemies. After finding the difficulty to be fine at first, I noticed just after the first few runs few runs, that suddenly level 1 spawned much harder enemies then on the runs before. That equals a form of difficulty scaling up with unlocks. It has been known in the community for a long time too.

According to an old interview with the developers, each level has a selection of rooms to draw on, but they're graded on difficulty and the map generator won't place the harder ones until you've experienced the easier variations first, so you can gradually get used to the concepts. So the more you play, the more difficult rooms you're unlocking. source

When I get into a game I want to get to know a certain difficulty setting before I turn it up. The sudden change in enemy types I experienced in EtG after the first runs made me feel like playing a game that arbitrarily changes difficulty without giving me a choice over it.

Of course my impression might be wrong. Do enemy types per level/area turn consistent later on in the game, or do they keep changing with unlocks?