The pros of using a static rendering method like in Blender is that you can render every frame with full quality, which means global illumination and stuff, you can spend hours on every frame to make the lighting perfect. For animation you can also use rigs which use blender specific stuff like tons of constraints, for example the fancy new Blenrig 5.
The cons of this method is that it can take a long time to render. If you’ve got a fast GPU it can be faster but you’re still probably looking at several minutes per frame. Very simple scenes can maybe have frame renders in seconds but then you’re not getting the benefit of the realistic aspect anyway.
If you were to use UE4 the obvious cons would be that you would have to use rigs that work with the engine, which means no fancy Blender Blenrig (or similar) stuff. If you scale a parent bone all child bones will scale with it, so no disabling “inherit scale” or anything. The other con is that scenes will perhaps look less realistic than static render methods, but UE4 will probably look pretty good when the settings are cranked.
The pros of using something like UE4 is that it’s super fast. If you’ve got a beefy machine you can easily reach 60 fps (or more), and then you can render your animation at more than realtime speeds. I remember doing some tests in UE4 rendering small resolution animations and I got 200 fps or something, it was hilarious. This realtime feedback is also in place when you set up your scene, so it’s much more intuitive when setting up lighting and so on since you don’t have to wait for renders, you can just change parameters on the fly which is pretty mindblowing when you try it out. For the final animation you can crank the settings when rendering, so even if your machine can’t handle it in realtime, it will render much faster (at least several FPS compared to minutes/hours) than a similar scene in Blender.
Another pro is that you can use engine specific stuff in realtime too, like ragdoll physics, various simulations (like APEX for clothes or the new dynamic animation thing), particle effects and so on. I forgot about this but UE4 is based on a PBR shading model which means that materials will look very realistic, and you can also use free materials called substances (with a plugin) which are materials made in Substance Designer or Substance Painter. Or make a fancy shader by yourself if you want to, you make those with nodes as with cycles.
The UE4 devs are right now working on a new system for cinematic animation called Sequencer which is pretty much what you’re looking for. It’s a work in progress but can be used already and should be even better in the next release. I’m pretty interested in this myself and think it’s certainly viable for certain types of animations. The best thing is that even though UE4 has a royalty model where you pay them 5% of your revenue this is only true for games. Movies/videos are completely free.
An example of what Sequencer can do: https://www.youtube.com/watch?v=3OCJCZJWA68
Edit: Worth pointing out, you don’t only use UE4, you would use Blender to make your models, animations and stuff, then bring that into the engine.