As for resent release of Vala 0.39.4, there are huge improvements if we talk about warnings output at Vala code and C code compilation level.
One of the argument against Vala, has been the number of warnings you get for a valid Vala code at C level compilation. As an example you can check warnings for GXml in March 2017 about 230, some were my fault but other at C level.
Resent rebuilt in January 2018 for GXml shows a drop to just 9 warnings. But the extraordinary thing is that no C level warnings at all! Remaining warnings are for missing features in GIR format and one from Vala code.
Hope to find time to publish resent Vala improvements this 0.40 development cycle.