I'm using Vivaldi since first public release and I haven't encountered any issue regarding to Blink stability.
I'm using only newest snapshots (so they should be considered unstable) as my daily work & private browser and never had a reason to roll back. IIRC they using even nightlies from Blink in their snapshots, so "getting behind" shouldn't be considered a problem I think.
But putting everything aside, let's assume they're 40 days behind Blink always - are you THAT geeky that you need these new Blink features instantly, when noone actually implements them anywhere (I'm taking about new JS stuff for example), because noone would be able to use this features, as this is cutting-edge-blink-only-feature?