Topic: UFE and 2D sprites
Here's a writeup on how I use 2D sprites in UFE. Hopefully this can help get you started.
Tools used:
They're probably all optional; just posting what I used.
Anima2D - I use this for the bone structure. Probably not even necessary but it's free and helps visually so whatever.
Sprite Illuminator - I use this to create normal maps for our sprites. We're using digitized actors so helps them look less flat.
Texture Packer - Works with Sprite Illuminator and packs everything into spritesheets and materials. Also has a Unity import plugin.
Double Sided shader - I have one that came with a weather plugin, but looks like there's some free ones on the Asset Store. I don't use this since we don't mirror sprites in our game but generally that's how it's done so you'll want to grab this.
1. Start the prefab
Create an empty object and give it your character name. The UFE prefabs are set to -2, 0, 0 postion and 0, 90, 0 rotation, so go with that.
Create a child object and name it 'Render'. Add the Sprite Renderer component to it. Drag your first idle sprite into the Sprite section. You'll want to align the bottom of your character with the grid line at y=0. You can always adjust this during animation, but if your sprites all line up the same you won't have to.
2. Create the bones
If you have Anima2D, right click on your main object, 2d, and select Bone. Drag the handle point of the new object up to your characters chest, and rename it Torso. Keep in mind the hitboxes are spawned at at the handle point; you can ignore the other end, it's really just there to help you visualize the bone structure. If you don't have Anima, you can just create empty objects and place them where you want, but having the bones to grab during animation is pretty handy.
Keep right clicking and adding bones, positioning them wherever you want hitboxes. The obvious ones are the chest and head, and for the rest I tend to place them in the middle of each body part. You'll see here I have them in the middle of the bicep, forearm, thigh, shin, and then directly on the hands and feet. You can fix the parenting of the bones in case they don't link the way you want. Rename each one appropriately so you can find it later on.
Keep in mind, the UFE charactes are massive in size. You may want to scale up your prefab/render accordingly, otherwise they'll look tiny against UFE characters. 
3. Add the Hitboxes script to your main object, create your character file, and assign the hitboxes like you normally would. At this point, you can load your character into the game but we don't have any animations yet.
4. Animate
Create an animation for your character. In the Animation window, Add Property, Sprite Renderer, scroll all the way to the bottom and select Sprite. Drag all of your sprites into the timeline and make sure it animates properly. Once your animation plays how you want, make sure the record button is enabled and start lining up your hitboxes according to the animation. Move the highest hitbox in the hierarchy first, otherwise you'll be constantly fixing the lower level boxes. You can try to use the interpolation to your advantage, but I've had some weird results in the past so be prepared to animate each frame if necessary. If you're using materials, you can add them on the first frame; it's called Material Reference under Sprite Renderer.
5. Go for it
This should be enough to get you started. Blending is not your friend with 2d sprites, so make sure it's always set to 0 or off. Adjust your material settings properly to get the best lighting. It's been a while since I set all this up, so may have forgotten something. Any questions just ask and I'll try to help. Good luck.
.I'm doing a 2d Fighting game with UFE by following this tutorial and because when I click "Play", hit boxes disappear and preview animation is no longer accessible. I do not understand :'(
