we plan to abandon any updates to legacy coreboot and put all our effort to make mainline fully-featured. We still do not have automated pfSense validation so you have to rely on your own testing.
We plan to publish validation results of each build to rise awarness of community about validated features.
If you have some use cases that you think should be verified please let us know and we will see how we can incorporate that in monthly regression. In that way quality of mainline firmware will be improved over time.
To summarize, if you need rocket-slid long time validated code base, you should use legacy. If you missing features or can risk little bit since firmware rollback is not an issue for you, then you can go with mainline.