about the 'plastic bag' comparison, my impression from the YKYWB and Darevie clothings is that the fabric is well-ventilated and wicks the moisture pretty well. For jerseys especially, and I even have one from YKYWB which is summer specific, and has a mesh fabric for the whole back and sleeves, which is really cool and fit for hot rides
But for bibs also, I don't have any issues with them being too hot (currently have 4 from YKYWB and 3 from Darevie), not even for indoor rides. But surprisingly they also hold the heat well in the coldest rides.
I cannot compare to Spexcel bibs since I have none. But the 3 jerseys I have from Spexcel, my impression is the fabric is hotter, slightly.