I think comparing Natron and Blender in terms of compositing isn't really a solid comparison. Don't get me wrong, Blender has a decent compositor and that's cool, but it's not really a compositing program. It's a program that can do it, but I wouldn't choose to use it over After Effects or Natron for most tasks. For 3D, it excels - after learning Cinema 4D and Maya, I came back to Blender every time, but for actual compositing Natron's biggest advantage is that that's what it's purposely designed to do. It has support for most OpenFX plugins, and does an excellent job with the tools it has. It's surprisingly stable for what it is, although there's room for improvement, and it has a reasonably intuitive workflow for compositing. I personally find the best way to work is do your 3D components in Blender, render out, and then bring it all together in Natron - use the tools for what they were built for, and you'll get the best results.
That said, when it comes to tracking, IIRC Natron uses Blender's tracking algorithms doesn't it? I'm not super experienced in Natron yet, tbh, but from my experience so far, I'd use Blender's tracker for 3D camera solves and for tracking elements into the scene before render, but Natron's tracking tools seem really solid for planar tracks and basic transforms. Not as fast as what I'm used to in After Effects, perhaps, but very good. Worth the slight tracking speed reduction to switch from a layer-based to a node-based workflow.
On a side note, gotta give props to Natron for EXRs - they seem to work effortlessly and with less mucking about than AE.