What's so disappointing about Just Cause 4 is not any one particular error, nor even any catalogue of errors, but its utter lack of ambition and unwillingness to innovate on a formula it has been milking for years. In fact, if JC4 had been released as DLC for JC3, it would still have been a let down.
The gameplay mechanics haven't evolved, Rico shoots around the tropical island with his trusty parachute, wingsuit and retracta-zipline or whatever it's called; any graphics bump is negligible if it's there at all; the landscapes and village environments look largely the same - which is to say, well rendered, but repetitive and empty; the characters, both allies and villains, are familiar and as two dimensional as ever, without even a sufficiently contrived motivation for Rico's mostly-missing-in-action arch nemesis Espinosa, who seems to have devised multiple methods of manipulating the weather yet can think of no better application for this miraculous feat than crafting dubiously effective weapons and terrorizing the island's locals. There's some lofty and nebulous (pun intended) talk of 'The Agency' and of a possible distant masterplan, but any substance is reserved for the game's inevitable sequel.
Presumably developers Avalanche Studios are banking on gamers finding hours of entertainment in the exact same ways they have previously, like launching cows and goats into the sky, surfing fighter jets and cars, and detonating just about everything. They've added some tricks to this end, giving a bit more control over your ripchord-grappling-hook, like exploding air balloons to lift things up, variable power boosters to propel things in different directions and different velocities, yadayadayada.
It's all pretty par for the course, and most of the customisation ultimately proves too laborious to actually bother with. There are stunt missions, and wingsuit missions, and racing missions and everything else you'd expect, but none of it offers any more entertainment value than watching a Fast and Furious movie on repeat. The script is bad, jokes unfunny and animations wooden. Capturing territories is more tedious and monotonous than it was in Far Cry 2, 3, 4 and 5 (combined), and the mission formulas loop so often, it seems the game devs took inspiration from the tornados they presumably spent all their time constructing.
If this is your first rodeo with Rico in the Just Cause universe, I'm sure it'll be a blast, at least for a few hours, but if you're coming back for more hoping for a compelling and expansive iteration, don't bother, this is the gaming equivalent of a new flavour of pot noodle, only probably less fulfilling.