Do the Blackbird ukuleles count as "plastic ukuleles"?
I live in a moderate climate, but low humidity (in the winter) has always been an issue for my wooden instruments. Floor heating doesn't help. We don't typically use room/house-wide humidifiers here, so every winter I babysit my ukuleles and the guitar. A few years ago I got a Kiwaya/Famous laminated uke, which I can usually leave out. The fretboard still seems to shrink a bit.
A plastic ukulele would completely remove any worries, but I've only actually tried two of them in person: the Kala Waterman and the Blackbird Farallon. The Waterman was really bad. It wasn't just the dull, lifeless "strings over a bucket" sound, which was expected, but the intonation was off already in the third fret. That should not happen with a molded instrument. That was my first and last experience with Kala, probably unfairly so!
The Farallon had a very different price point and a lot of people really like it. I unfortunately did not and returned it to the dealer. Flaws in the material and globs of glue inside the instrument aside, the tone didn't do it for me. It also sounded bucket-y. I was actually really surprised by this, but I've had wooden ukes that cost a quarter of the Farallon's price that sounded far better to me. Playability was great, though, so this is probably just a matter of preference (amplified by the fact that I generally never really warmed up to tenors and prefer either baritone or the smaller sizes). I'd still like to try out a Blackbird Clara at some point.
As for "Isn't there enough plastic already?", I think yes, plastic should be avoided when possible, but I don't believe a single plastic instrument that may well be used for many years is a big concern when compared to how much daily plastic trash everyone is responsible for (from yogurt containers to string packaging). A single plastic ukulele also probably has a lower negative environmental impact than owning 5 or 10 wooden ukuleles (energy to make and transport them, chemicals in the finish, plastic trash from shipping packaging, etc).