Subnautica Below Zero is definitely in very early alpha - I bought it to support the studio, much like I bought Subnautica in early access (back when you could stick solar panels on your Cyclops sub to recharge its batteries, and there was no story) - but I'm holding off playing Below Zero until it's done.
My support of Subnautica was worth it, such an amazing game at the end, I don't expect Below Zero to be any different when its finished.
Also - the studio released this game like they did Subnautica, with feedback to the developers built directly into the game, because it helped them deliver a high quality product.