I think "it depends".
These pants look very fun BTW. Track pant styles, of the right type, can work really well for me and I wish I could have a couple of them, or trackpant-esque ones, that stay in style and are work-appropriate.
Viscose is a type of rayon or maybe just the same thing, I'm not expert enough to be sure, so it is made from an natural material originally, but the weaves can be different amongst rayon items. So it usually feels more breathable than polyester. I have the Leith track pants, which are all poly and they would not be comfortable in August running around outside, but would work in the frigid A/C! I have some rayon or rayon blend loungewear pants I wear around the house in summer--we don't keep home A/C arctic at all, but I often like something more than shorts.
The thing will be the washability. These could be technically washable, but delicate. Hand wash or delicate. Air dry usually--heat is bad for rayons. Shrinkage is always a problem--sometimes sneaks up on you. Wrinkles can be a problem though the spandex may help that; most 100% rayon needs ironing to look its best, say, to look like it did in the store! So, you might have to iron after washing to restore the drapey look. So, if you are wearing and getting a little sweaty and thinking of freqently washing them the same way as some cotton casual chinos or jeans, they are not. But, EF does some beautiful fabrics and may have made these less fussy. Also you may be thinking of grocery store, errands in air-conditoned car and not spending much time in brutal heat, so they would be fine for that.
I love rayon shirts and blouses because they often have gorgeous color and they also drape beautifully, which lets me fit in a little ease and "fluid fit" which is truly fluid, not overwhelming, But then, if 100% rayon, they may need ironing , so I often pass them up OR look for simple styles I could touch-up iron easily, not have to do a lot of collars and cuffs and such.
Rayon and polyester blends bring in some of the downside of poly but wrinkle less.
There is a previous YLF thread on such fabrics and maybe more than one.