Took some runs at the demo, here's some information and some impressions I've had.
The demo consists of a few tutorial fights and then opens up into the Cornia area of the game. This is the... prologue? Chapter 1? It's never identified as such, but I'll be referring to it as the prologue (maybe in the full release it'll be called different). Also, don't let the "tutorial" moniker fool you - if you're playing on the highest difficulty and not paying attention or playing well, you can get worked over by the last tutorial fight. The tutorial phase will probably take about an hour and once that ends you've basically got full run of the country of Cornia and a 5-hour time limit will begin. The demo will end when either the 5 hours are up or you've defeated the main story boss of Cornia, whichever comes first. The timer ending will not necessarily stop you where you are, if you're in the middle of a map battle it'll let you finish it and then the demo will end (so feel free to start the final boss with 30 seconds to go). I mentioned in a previous post, when you're in menus the timer pauses, so you can fiddle around with formations, equipment, anything you want in the menu and not feel any kind of time pressure. Also, the 5 hours is NOT tied to the demo itself, only a playthrough. Meaning you can finish a playthrough and start another one if you'd like. It's also tied to when you saved, so if you've been having trouble on a stage and lost half an hour, you can reload to before the stage started and get all that time back on the timer. After the final tutorial fight, you can make a base save and do repeat playthroughs from there to save time. The prologue has A LOT of content, so 5 hours may not even get you through half of what's available to you on a first playthrough. A lot of time will be spent watching cutscenes and dialogues - my first playthrough I watched everything, and then did so again skipping everything I could (and got about twice as far).
You really do have access to the nearly every area in the prologue right from the start. You can charge over to the prologue boss immediately if you want, though you'll probably get smoked. The way the game hems you in is through enemies roaming the map in unliberated areas: if a roaming enemy bumps into you then you'll engage in a normal fight, though you can choose any one of your unit formations to face them. If you lose, you're booted back to the closest friendly area (with no other penalty). If you win, the enemy is stunned for a bit and you can keep going. If you can kill them completely in one round of combat then they disappear from the map entirely. Completing a nearby map will liberate the area and clear it of enemies, letting you explore freely. This system is what generates the risk/reward of exploring; you can go much further than where the game expects you to be and potentially grab some nice stuff, like materials, equipment, sometimes even characters, but you may run into enemies 10 levels higher than you and be booted at any time. You can slowly build up your levels and forces through nearby maps, or crash into areas higher than your level for some extra challenge and rewards. Normally after clearing Cornia you'd have access to the whole world in whatever order you'd want, but that's not available in the demo, only the full release.
The gameplay itself is very similar to Ogre Battle, which the developers have been upfront about as one of the major inspirations. You'll spend a good amount of time fiddling with your unit formations and figuring out how to optimize them. Once engaged in battle a unit formation will act automatically based on the actions available to them and what direction you've give them (more on that soon). You start with 3 potential formations of 2 units each, though you can eventually unlock up to 10 formations of up to 5 units each through "Honor" points and your "Renown" level, which you get from completing maps and fulfilling some quests (mostly it'll be through battle). You won't be able to unlock that far in the demo, but pretty quickly you'll have 4-5 formations of 3 units each. By the end of the demo you may've unlocked a 4th unit for several of your formations. I actually like this easing into formation system, as it forces you to figure out simple mechanics and synergies before the game gets too complicated. What constitutes a "good" formation will change constantly depending on what's available to you and what you're up against and on higher difficulties you'll need to remain flexible and be ready to try new things. In the first few maps I had a strong "kill shit" team who steamrolled everything. Then they started running into armor knights and thieves and suddenly couldn't do anything.
The last Ogre Battle with this system came out about 25 years ago, so they've done plenty to innovate the gameplay. The biggest addition is a sort of "gambit system" where you can set conditionals for your units' actions and can drastically change the way a battle plays out. There are dozens of potential conditionals, from what kind of unit you'll prioritize, to hp thresholds that have to hit before they'll fire, which row they'll prioritize, etc. A good example of having to manage this is with the unit Josef, a strong pre-promote you begin the game with (he can take on most early game challenges, but won't gain levels and get stronger like the other units). He has a very strong attack, but is also capable of using his action for healing. If you don't set good direction for him, he might waste his actions healing units that are still above 90% hp - or he'll let his formation dwindle away to nothing when they could easily manage enemies on their own with his healing to support them. It isn't very complicated at the start of the game, but as your units grow and gain more possible actions, there's a lot of strategy available to you. Takes some getting used to, but once you get the hang of it, it can be very fun. You can probably ignore this on lower difficulties, but on higher ones it's something you can really play around with. I've heard this system called a "programmers dream" and seen people post some bonkers pictures of 5 different sets of priorities which may go into activating a single possible attack. Feel free to go nuts with it.
I've heard some complaints that people would prefer to manage a unit's actions manually, like something closer to the old Final Fantasy, but I don't think that would work very well with this system. A typical battle map may take 10-20 minutes (maybe close 30 for a huge one). An individual battle will take about 10-20 seconds - you can speed that up to double time if you want, or just skip it entirely and see the results. The gambit system is done to save time; if you were inputting manually, each battle would probably now take a minute or two. A 10-20 minute map would now take up to or over an hour. The game would suddenly balloon to 400 hours. Someone may mod the game to unlock that way down the line - and if you really want you can manually change each units' attack priority before each battle - but in the meantime just think of yourself as the General of this army instead of a soldier in it.
If any part of that sounded good, I'd encourage people to give it a look. The demo's free, is now out on (I think) all of the platforms it's releasing on, and it even emulates well. You'll likely get over 6 hours of time to play around with it and see if it has anything to offer you. And despite the doofy fucking name they've given it (Unicorn Overlord? Did that sound better in Japanese?) I think it's pretty good.