Unwelcome online features
Of the various technical and game design issues JC3 suffers from, I think the obtrusive online features (i.e. the leaderboards) may be the worst. I have hard time believing that there exists a single person that gives a flying fuck another completely random person has gotten a higher score in some arbitrary activity. And that it needs a text feed in the game with a sound effect for every new line.
That would be a fairly minor annoyance alone but then there is the thing with how the whole leaderboard syncing process is oddly prioritized to the level it causes hitches in performance. It goes even so far that the whole game freaking pauses if you happen to lose connection to Square Enix's servers until it reconnects. If the game is then unable to reconnect (which is likely to happen) you can retry or go into an offline mode. The latter is however only a momentary respite as the next time you open the map screen (via which everything is accessed) the game will try to reconnect again! And that is the only place you can choose to go offline. (Launching Steam in offline mode might work too.)
Lacks some optimization
JC3 also has performance issues. My PC meets the listed recommended system requirements on CPU and RAM, and exceeds them on GPU. That is apparently not a guarantee for the game to run at stable 60 FPS on 1080p at max settings, although it did so most of the time.
Heavy action especially in towns with many civilians in addition to the military would often cause framerate to drop. Then there were locations where simply looking at distant scenery caused low FPS and vsync to kick in to lock it down to 30. I disabled vsync around 75% through the game. That caused some tearing at places but the game in general seemed to run moderately better after that.
There is a low performance warning at the start nowadays if you do not have enough system memory. I am not sure what the game would be satisfied with but 8GB is not enough. People report that 16+ GB of RAM does help with performance, though.
Having JC3 installed on an SSD might hasten the many loading screens you get during story missions too. Many of them seem so unnecessary. Like, surely showing a squad of enemies getting into an ambushing position was not enough to warrant a loading screen? JC3's inital boot up time is also rather extensive. And according to some forum comment an SSD does not do much for that.
Many small features removed
Gameplay in JC3 is familiar for anyone coming for the previous title. However, one quickly starts to notice these little things missing. What happened to sprinting for instance? It was not much faster than normal movement in JC2 but it was still faster. Combat rolls are also gone. No more strafe-dodging into cover I guess. No crouching either. I guess they wanted players to just use the damn grappling hook all the time. Minimap is gone as well. Hell, even weapon zoom was no where to be found, though I later learned it still existed -- it just had to be unlocked first by doing weapon challenges. The weapon zoom is a bit buggy as well -- it often likes to disable homing for certain weapons. This makes one story mission impossible to complete until you disable the mod and restart the game.
Upgrades behind a challenge wall
While I appreciate Avalanche for ditching the boring upgrade system, I am not sure if it was such a good idea to put everything behind completing various challenges and making the unlocks happen in a set order instead of giving you freedom to choose what you want first. A lot of the upgrades are simple, like more grenades and longer vehicle boost, but some of them also alter stuff. And they can be disabled if you do not want your grenades to home in on enemies anymore, for instance.
Changes to movement controls
Surprisingly enough air races were not all that difficult like they were in JC2. Avalanche added somewhat requested feature of rudder controls to the planes and that made flying them so much more easier. The crash bomb challenges can be annoying due to the seemingly random way the vehicle acts after you have abandoned it. But the prize for the most challenging of challenges go to the all new wingsuit without a doubt.
I have kind of mastered the wingsuit now after 70 hours and now looking back at the challenges, they do not seem as difficult. But a few hours into the game, 5-gearing some of them seemed nigh impossible. The most trouble I probably had with was one that ended in an underground military base. I do not even dare to guess how many attempts it took to master the final tunnel.
In addition to the planes, land vehicles also had their controls slightly altered for the third game, although I do not agree with the general consensus that motorcycles became worse. To me they seemed the same and fine. Cars do not like turning at all, however. Mastering drifting with handbrake is required to make tight turns at high speeds. It is almost ridiculous how trying to turn normally does not seem to do anything but then a little tap to the handbrake and you are already going to a different direction. Getting the F1 car makes many of the land races much easier due to its great handling and speed.
Panau was better
Medici is sadly less diverse than Panau as well. There are fewer biomes and unique locations. The villages lack genii loci -- they all feel like the same cliff-side Mediterranean town. The south side of the large main island has settlements but the north is simply barren outside of few outposts and military bases. The capital city is pathetic in its size compared to Panau's. And even though the whole map is almost exactly the same size as in JC2, it has a whole lot more water in it.
Some improvements too
Destructibles are also not thrown everywhere in the world but are almost always within a settlement. And they start becoming visible on the map once enough of a settlement's objects have been destroyed. No more searching for that single little transformer hidden between two buildings.
The heat system is similar to JC2 and it can get as annoying at high levels if you are trying to get away and the game just keeps spawning enemies everywhere around you. However, JC3 has monasteries that are able clear the heat when visited. You can unlock them by completing random world encounters (which seem to be the buggiest feature the game has with how they rather often fail to complete). Enemies calling in reinforcement also have an icon appear over them, and taking them out quickly enough stops more enemies appearing. I like that the player has more control over the system like that.
Season pass probably worth it
The three bigger pieces of DLC that are included in the XL Edition add each an area to the world map. They have a very loosely connected story and the content is not really as good as the base game's. Their main purpose really seems to be to give you access to some truly powerful stuff.
Sky Fortress gives Rico a Bavarium powered wingsuit that allows infinite flying (and it also has machine guns and homing missiles, just 'cause). Mech Land Assault has a very powerful mech vehicle for you or a supporting rebel to drive.
And finally, Bavarium Sea Heist gives you a missile boat and the eDEN Spark. After fully upgrading the latter via its challenge, you will have a practically unlimited orbital laser at your command. It reminds me of Dawn of War's orbital bombardment or Independence Day's alien laser. It is about the most ridiculous thing I have seen in a video game. Although, I have to say that the M488 "nuke launcher" from the main game is pretty great too -- it destroys about half an oil rig's chaos objects with a single shot.
I do love the doge filter, though. Oh yeah, the soundtrack is great too. I enjoyed Henry Jackman's work in the Kick-Ass movies and he composed well for JC3 too. There are many amazing tracks in the game.