No, it isn't really that bad. People love to hate it because that is the cool thing to do. That isn't to say that it is a perfect or great game (as all of the complaints that you've heard about it are based in reality), but it isn't really a bad Final Fantasy game. The trilogy is comprised of three pretty decent games. The second learned from the first and the third doubled down on doing whatever the fuck it wanted to do.
The first half of the game is linear, like most Final Fantasy games. It is infinitely more noticeable, however, as you cannot backtrack, there is no world map, there are no towns, and there are no minigames. In Final Fantasy X, you ran forward to the next story exposition and repeated that after.
I imagine that "Press X to Combat" is possible for the first part of the game as well, but I actually liked the combat system more than the previous games as there was a more active component. The system lacks a lot of true depth, especially once you figure out the best party setups. It isn't a hard game unless the stick by which you measure it is the easiest game in the universe, but I still found it enjoyable enough (especially considering I was expecting something that had me a lot less active).
The character progression system isn't my favorite. Each character as different abilities in the same trees, so two characters of the same class will be more/less useful. Variation itself isn't bad (set character archetypes is perfectly fine), but the fact that it looks similar to the sphere grid gives a false impression of what it is. You grind for points, max the level you're at, and go up another level. You don't make any actual decisions within the tree. Much closer to FF4/9 (character archetypes) than FF6/7/8/10 (blank-ish slate).
The story is forgettable, but as I've mentioned before, I'm not a fan of the stories in Final Fantasy games. A lot of them have great overarching ideas and themes, but the implementation is subpar at best (but that can be said about nearly any story written). Execution matters, and it shows here. The team that came up with the actual lore surrounding everything that happens in 13 did a pretty good job. It was interesting and intriguing...except it is hidden in menus instead of being part of the world. I imagine that while the team was off trying to make this pretty engine, the writers were writing as much as they could. When it came time to implement it, we ended up getting a section of the menu for story. Oh well. The plot of the second game is so disjointed from the first that it is obvious they needed to make another game in the series for money (or perhaps Toriyama fan fiction) and not because they thought it was a good creative decision.
The characters are fairly stereotypical and generic. Most people point to Sazh as the best and complain about Vanille's accent and Hope's dialogue, but all the dialogue reads like a translation. The actors themselves do a good job with the material they're given, but there's only so many ways you can repeat the same lines without sounding like an idiot. I had no problem with Vanille's accent. I thought that Hope's motivation for being a little shit was contrived, but all kids are little shits so it was pretty easy to ignore. Fang should've been onscreen more and needed a sex scene with Vanille. Snow is given a stoic front, but they don't do enough to realize what's happening underneath that. Sazh probably is the best, because he's the most likable of the cast, but I feel that they missed a huge opportunity with his character to do something really special. Lightning's flatness is a foil to Snow and the motivation is (as with Snow) underutilized. Overall, the entire cast needed more in terms of progression and more measured. Lightning Returns gives them a bit of progression, but 13-2 doesn't do much for the leads.
I actually disagree with the idea that the game opens up at any point. Even when it does, the content that is there is just a bit of battle content that is option and occurs in a more open area. It would have been interesting if the linear portion of the game had been a much shorter intro segment with an expanded open portion that integrated the story of the land into it (which is what I thought people meant when they said it opened up). So remember that when people say it opens up, they really mean that you have one activity to accomplish at various waypoints around the area.
I completely understand why someone wouldn't like the game. There are a huge number of poor choices made in the design of the game that I would not have made. What I don't understand how those people liked many of the other Final Fantasy games based on the complaints that they have when these same flaws are present in those games as well. Maybe it just hit the tipping point for the bad design present in the other games?