A Splat is kind of like a billboard. It’s a way of using a 2D shape in a 3D environment. Kind of like a billboard. There are some differences in how they work. For instance, if I recall correctly, the Splat is automatically the correct aspect for your image, whereas if you make a billboard, you need to make sure the plane is the correct aspect. There are some other features as well, but I don’t remember them all. I tend to make my own billboards, as a drawback I’ve had with Splats is that the image is in the center of a bounding box, and the edges of the bounding box don’t correspond with the image. There’s also some shadow wonkiness sometimes.
As to rendering and animations, the same limitations would apply to using Splats and Billboards. Both are fairly easy on resources, so play around a bit in a simple scene and see which you prefer.