During Picard's reign, the Enterprise was mostly running peaceful exploratory missions. Now you're in a pre-war scenario and there's a ton at stake. Jellico had to get the shipping running like a military warship, rather than a science or diplomatic vessel. Riker wasn't used to that. Maybe he'd gotten a bit fat on his laurels.
I'm never too kind to Riker for a multitude of reasons, but I'd be willing to come to his defense here. The man successfully engaged the Borg. He had to have been around during the Cardassian war. Even though he probably wasn't the best candidate to handle this negotiation, I'm still willing to say he'd be up to handling being on board a war time vessel
His problem was that the new captain offered no professional courtesy to him beyond the 30 second introduction. Riker, just like everybody else, had to just shut up & deliver the demands of the mission on a nearly impossible scale, with very little input, & absolutely no excuses. Ultimately, THAT is what he's not use to. He had grown to expect that his input was not only welcomed, but preordained. It is not, certainly not when the mission is laid out in absolute specific detail. It was a hard pill for him to swallow
His mistake was that he assumed that this was all due to Jellico's way of doing things & not perhaps a prerequisite of the mission itself. Frankly, I'm of the opinion that Jellico & perhaps with Starfleet's knowledge believed that this was the only way the mission could be done, right down to inevitably ruffling feathers on board the Enterprise
It's insane to think that he'd be the kind of hardass that forced every order throughout his entire career. Who could work like that? This circumstance is a unique situation with very little wiggle room, & being a jerk was the hand he was dealt
Riker, & everybody else for that matter, never stopped to consider that. They just assumed Jellico was unreasonable, just like Lemec assumed he was unreasonable. "Unreasonable" was his mission objective. It was the only way to make this work, & it did