There's previously been efforts to use more Rust code in GNOME than C/Vala and developers expressing their disappointment/frustrations in Vala. Emmanuele Bassi recently tweeted, "PSA: if you want to write a new @gnome application, don't use Vala; if you're already using it, consider porting to a non-dead language."
He went on to write, "Vala was a neat idea in the beginning, and new languages popping up in the last 5 years kind of validated that approach...Sadly, it never got to the point of being developed by more than 1 person, and reaching a symbolic '1.0'...And it's still without any form of support in terms of tooling and build system integration that doesn't make baby seals cry...So, tl;dr: do not use Vala for new projects. Thank all the contributors to the language (they deserve it) and then move on."
Following some disagreeing with his assessment of Vala, Bassi has written a long blog post pleading his case that there either needs to be significant investments into Vala or that new GNOME project efforts should be making use of other languages. Those wishing to read his thoughts on Vala can do so via his blog.