The next major version is going to be out of testing not before a month I would say. The whole engine has been re-written and it should be very optimized for Tiled and MipMaped EXR files (to support fast workflows even with very large size images).
The issue with scan-lines is that the only software currently taking advantage of it is Nuke because internally they just process scan-lines.
In Natron we require tiles (i.e rectangles), and reading tiles from a scan-line file is much less memory efficient. Reading with tiles allow to keep a very small portion of the image in memory and is a lot easier for very large images (i.e 4K or more).
From my experience I'm aware of studios working only with tiled EXR files. They have a special conversion step for Nuke where they convert everything to scan-lines and back to tiled in output. This is a real bottleneck.
The preferred format for any software should be tiled and mipmapped, this is the only format that can be read fast and with low memory regardless of the image size.