Product review (5 stars): Challenging but fun; consider the Cruiser or Grom
I have the Freeline Grom, which I liked so much that I got the OG as well.  First I'll discuss Freeline skates in general, then I'll discuss the difference between Grom and OG.

The OG is very tough.  The deck and truck is one piece cast aircraft-grade aluminum.  It can bear up to 3,000 lbs.  My minor complaint is that the griptape isn't very grippy.  I replaced mine with Vicious Black griptape which I'm pleased with.

Learning to ride Freelines is not as difficult as it may seem. In terms of difficulty, learning Freelines is definitely harder than a casterboard but I found it easier to learn than Orbitwheels or the Skatecycle. It just takes persistence, and a little thought. The trucks don't move so you need to pivot (more like steer) the skates in and out. You should also coordinate the pivoting with the direction that you push each skate. For example, if you're riding regular, and your toes are pointed in, then your left/front leg should push "up" (from a birds-eye view) while your right/rear leg should push "down" and vice-versa. It took me 3 days of 2 hours each to learn the Freeline to the point of not needing a wall to launch or propel myself, and be able to turn and stop. To be fair, I had been practicing with Orbitwheels for 8 hours total before that.

I found Freelines to be fun to ride. Downhill, it feels a lot like snowboarding, except your feet move independently. Some skates feel squirrelly going downhill, but the Freelines feel stable and I have confidence in their traction. On flats, they also feel good. However, on flats, you do need to maintain a brisk walking speed, or else they will feel hard to pivot. That requires learning techniques to start moving from a standstill ("zero start"). I found FLSTeamBushi's video tutorials most helpful.

Freeline skates do take more strength and endurance to ride than let's say inline skating. On inline skates, I can skate for a long while before needing to stop. On the OG, I can currently skate for only a couple of minutes before needing to pause. At that point, the soles of my feet are sore, and my quads are burning (as though I did one long set of many reps). Hopefully, as my technique improves and my muscles grow stronger, I can skate longer.

As expected, Freelines feel best on smooth surfaces. You can still ride them on rough asphalt, but it's much harder to propel them.

BTW, I also got a pair of fake Freeline skates (branded Sui Xin). The deck looks tough enough (aluminum, but not the same kind as the OG) and it even has optional straps, and flashing wheels. However, the wheels and bearings are absolutely horrible. It takes far more effort to propel them, to the point where I can only go for a few feet (whereas on the Groms and OG, I can skate for a couple of minutes before stopping). They were so bad that I tried changing the bearings to Bones Reds. It was a huge improvement but one of the wheels still would not turn smoothly, and the other wheels would turn only about as much as the Groms' wheels (on normal wheels, Bones Reds should spin more smoothly than ABEC5). With bearings replaced, I can skate only about 30 seconds on the fake Freelines before running out of momentum and leg strength. It still takes much more effort, and it's not as fun as riding the OG or Groms.

GROM VS. OG VS. CRUISER
As mentioned, I started out with the Grom and now have the OG as well. I can confirm that the Grom is much easier to propel than the OG. I wasn't sure why (is it the decks? or the wheels? or something else?). I put the OG wheels on the Groms (yes they do fit). I found that with the OG wheels, the modified Grom is actually better. It rolls more smoothly, and it is also a little easier to stay balanced on it. Based on my experiment, it appears that it is the Grom's deck that makes it easier to ride than the OG. That brings me to the Cruiser: I saw a site that showed the Cruiser wheels being 72mm (the size of the OG wheels). If so, then the Cruiser should be better than the Grom because it has the same deck, but has the OG wheels.  I haven't tried the Pro, but because its deck is wood, I suspect it is also easier to propel than the Grom, albeit less durable.