Wrong on the second count! You can never get all of your vassal levies and vassal's vassal levies etc., and adventurers are based on the sum total of all realm troops. This is how people keep reporting 30k adventurer stacks when the Byzantine Empire or the Abassids are unable to field 2/3 of that, and they aren't even playing as one of those two.
Thus, in a scenario where your realm is sizeable but not huge, or lots of delegation is going on, you can quite easily get hugely outnumbered by adventurers. :/
Sounds like bad management to me. If you are unable to raise sufficient troops from your realm, then you obviously are not a good ruler. This is where the whole de jure system comes in and allows adventurers to break up realms which have expanded beyond their de jure borders.
Like others, I have yet to have problems with adventurers. You should have two years warning - with that warning, not being at war, and having money for mercs should not be hard (and if you are begrudging the money for mercs, well the guys got 500 gold why do you think he has it?). Of course, you do risk being mugged if your realm is comparatively weak & there are some situations that seem to spawn adventurer hordes that are out of whack (like the guy that got hit by about 20k because he had 3 spanish kingdom titles).
I will add, too many people who complain about adventurers seem to have an enthusiasm for using the console. To be frank, they probably should learn to deal with them.
No, you are wrong. They are capped. Easy to notice when the stacks are the same size as you get bigger. In the event, judging by his comments he isn't getting adventurers (adventurers can't be landed) he's getting decadence revolts (which is also a broken mechanic, but at least one that is clearly defined as a fail state).
I too see nothing wrong with them, but I always do my best to keep my de jure borders perfect and drift everything into my primary title. However there are several different adventurers hosts it seems. Random adventurer hosts from - aka non claimants - they are weak and you don't even need to raise your levies to defeat them a group of mercs and your set. Claimant hosts those that have claims on the realm they are invading seem to get much more troops, I just had someone leave my current realm to go attack their neighbouring relative, they spawned ~56k troops. I laughed pretty hard at it because I was waiting for enough piety to do the same, they then took a grand total of 1 county and didn't take the kingdom, duchy or the other 3 county claims he had. So there is now a small island province in the middle of this realm with 12k troops, capable of attacking everyone nearby and winning, but because borders it is not likely. 56k =/= 12k I know the aggressor was different religion and lost alot of troops to holy orders, I was watching closely incase I needed to lend a hand to win it for him. This in addition to the seljuks insta-spawning inside your realm with 0 warning with ~60k troops also shows that it is not limited to 30k troops for all situations. I later destroyed the remaining 50k+ troops of the seljuks with 30k troops, through much planning and baiting them perfectly.
I rarely see claimant host claims against my realm, because I actually land my family members if possible. Occassionally see small adventurers but they are never a problem, even for the AI. I'd say adventurer claims run about a 20% success chance and the claimants at about 50% against the AI. They are not that common, they are a feature of the game that adds alot of flavour and family management. I'd say rather than bitching about something in the game that is as far as anyone can tell is working fine, perhaps ask for help with how to deal with the issues.
I never assassinate adventurers, but for some reason it does seems that perhaps 30-50% never even actually launch against you. Best tip is fix your de jure borders, if you are outside this range expect to see people wanting to see this land given to someone else. Prepare properly war by saving cash. Learn to not wait for adventurers even, let them come, let them take a couple of holdings it weakens them more than you. They will split their stacks everytime, when one is a little away from the rest bait it away with a small army and crush it with reinforcements. Learn that the armies are typically 3 flanks divided into 3 armies, all in the centre. Stack your centre with defence and your flanks with damage. There are so many small easy tactics that can be used to win every war, they will not always work, but with practice and experience the AI can not win a war against you without outnumbering 3:1 or if you get really unlucky.
Also the disbanding of event troops is not a good idea. Unless they have a timebomb trigger that leaves them active until the land captured has lost the negatives to income and troops. You invade a region and disband your troops whilst surrounded by enemies, I think not.