I had a rather bad experience with an Adventurer the other day.
867 start, formed Volga Bulgaria, and I've taken a little bit of Rus and most of Perm. My total levy + retinue was maybe 8k troops. I had also just re-formed Suomenusko. Some independent count who I had just taken land off of is housing my future invader. This is a landlocked county completely surrounded by my own territories. The adventurer had a weak claim on Volga Bulgaria and went to town, as my years of plotting to assassinate him failed.
All of a sudden 25k troops spawn in the middle of my realm when he declares war (and right when my funds were fairly low as well). I was able to get through mostly through the AI's incompetence. I let him fan has stacks out a bit by letting him win a couple counties. Luckily for me, soon after I reformed Suomenusko the Holy Order appeared, and it just so happened the adventurer was Old Suomenusko. Combination of my troops and the holy order and baiting his stacks won the day (as well as Flat Terrain Masters), but it was absurdly lucky and gamey the whole time. The claimant even converted to the reformed faith during the war, but by then my numbers were superior and the holy order was unnecessary.
Point is, a guy from inside a landlocked region of my realm greatly outnumbered me, was allowed zero attrition, and did so with a weak claim. It was very frustrating to see.