There are quite a few things about RS2.6 that are bugging me at the moment. None of these are enough to make me stop playing, but they're still frustrating.
For one, the blatantly cheating AI drives me insane. When an enemy that consists of all of two provinces can produce and maintain multiple silver-chevroned stacks, and keeps sending them to attack my cities, it's both immersion-breaking and just plain annoying. Seeing several units of Agema Elite Phalangites suddenly pop up despite there being no real economic support is immensely aggravating.
The AI also has no concept of force concentration, especially during sea battles, where it'll dispense individual ships all over the place, whereupon my deceres can plow through them. It's only getting away with the same issue on land because it keeps getting handed free stacks with zero consequence, and thus their 'dispersed' armies are more like Oprah handing out veteran elite units.
Also, sieges. When armies specifically stated to not have siege equipment at the opening of the battle (because, yes, I read their army cards) are granted them for free, forcing my defenses to be spread out ridiculously, it's also immensely aggravating. If the sole difference between victory and defeat is whether or not the AI blatantly is given stuff it shouldn't logically have...
Maybe these issues, along with the outright broken diplomacy (which isn't your fault, granted), are fixed in RS3. I wouldn't know because I'm still working on my 2.6 Roman campaign.