Random adventures would be harder, but nothing impossible. For two reasons:
1) It would be slightly harder to prepare for the adventure, so you'd be forced to keep a larger army on standby to be prepared for anything. Get your max pop up high enough and this extra difficulty disappears completely. I'm pretty sure 3000+ max pop fits all the K, XB, E, C you might need for any camp, with a vast army of R/B/M as fodder. If adventure has 7-10 days time to complete, you can easily train anything you miss.
2) You can't use guide, so you have to calculate your own blocks and sim your own optimal setups. This is where the real extra difficulty is. Weakest players just send recruits on a hunch and pay the price. Best players know how to calculate perfectly safe blocks and end up saving lot of troops. Rest end up somewhere in between, making mistakes on blocks they thought were safe, suffering unexpected losses. That's real difficulty. Best players get rewarded for the extra skill and effort they put in, keeping them interested in the game. Meanwhile, weakest players might feel this adventure is too difficult for them to handle, so they simply avoid the random adventures. Overall, nobody should be forced to do a random adventure unless they feel it's worth their effort. Calculating the perfect setups and blocks isn't easy even for best players, so adventure should naturally have high rewards to compensate for the effort.
Requiring skill is real difficulty. Requiring you to send 3000R 3000B and 2000M to their deaths in order to beat enemy following a guide where nothing can go wrong is NOT.