So it's worth separating out two things here:
the qualities of rayon as a fabric, outside of any other consideration
the environmental impacts
This post is mostly about the first thing. A lot of companies are giving rayon many many different names as a way of disguising that It's Just Rayon, and claiming the fabric has special qualities.
But cellulose is cellulose. The process of extruding it into filaments and making those filaments into fibers/yarn/fabric is what gives it different qualities: some rayon is silky, some is fuzzy, etc.
It's all great at absorbing sweat, and it all takes longer to dry, and it insulates okay until it gets damp at which point it's worse than wearing nothing, which is why it's often blended into other things. The really nice tops I have from Uniqlo's Heattech line are a blend of a couple of synthetics and rayon. They're warm for being so thin and stretchy, but don't make me sweaty-feeling at all. (In a conversation among people with ADHD I found out I'm not the only one who wears them nearly daily for 3/4ths of the year lol.)
The irony of how often it's compared to polyester in the notes of this post is that polyester can also be made into a billion different textures. I have polyester that feels like wearing a plastic tarp, but I also own polyester that's light and breezy and totally comfy in boiling heat. I also have some very soft polyester fleece, as many people do. It's all a matter of how the filaments are extruded and how they're made into fabric.
But to get into the environmental stuff:
People get really into which fabrics are more "sustainable."
And rayon currently is made, 99% of the time, via one of two processes: viscose and lyocell (Tencel is a brand name for the lyocell process). Viscose is an older method and far more common, to the point that if a fabric doesn't specify that it's lyocell (or cuproammonium) you can probably assume it's viscose. Viscose is, generally speaking, far more polluting and hazardous to the humans working in the factory as well. Lyocell uses what's called a "closed-loop" method, so it puts out way fewer pollutants. It's also more expensive, generally speaking. There is such a thing as "ecoviscose" but I haven't looked into it.
(Modal just means rayon made from beech trees and afaict doesn't differentiate which process. Cupro is made using a less-common process called "cuproammonium," and I'm not sure how polluting it is, but apparently in China it's sometimes called "ammonia silk" which is wild.)
Rayon does have two definite advantages, despite everything I said up there:
you can make it out of any cellulose source, and that includes things that would otherwise be considered garbage/waste
it biodegrades pretty fast. Like, faster than cotton.
BUT THAT ALL SAID: every fabric requires something shitty, quite frankly. Cotton takes a TON of water and usually pesticides. Silk requires a lot of farming of mulberry and then electricity to warm the places where the silkworms live and also you have to cook the silkworms alive so they don't cut the fibers. Linen requires its own chemical soup to be turned into usable fibers unless you're making it from flax the old fashioned way which requires a lot of time and a shit-ton of effort. (Like seriously there's rippling, retting, breaking, scutching, and hackling. And THEN you can spin it into thread.) Wool requires a lot of land etc for sheep, but also any wool item you own that's machine washable has had the barbs melted off the fibers with chemicals, and in many cases is also coated with a resin!
And that's not getting into dying. But if you've ever dyed fabric at home you know that it usually requires careful handling and in many cases goggles. Those chemicals are often toxic as fuck.
If you're trying to be sustainable in your clothing choices, the fact is that the absolute best thing you can do is:
BUY LESS CLOTHES. Period. End of story.
Buy secondhand when you can.
Make those clothes last: use cold water washes and don't put them in the dryer and don't use fabric softener. Repair them when you can, and use them for rags when they wear out.
"What fiber is it made of" just matters way fucking less than buying fewer items of clothing and using them until they wear out.
But most people don't want to do those things. They want to know which brand of clothes is "sustainable."
The sustainable thing is to buy and throw away less clothes. That's it.
