In the steam curing process high temperature water is circulated through the pipe to cause the resin compound to catalyze (industry term is "kick"). Steam is not allowed to escape from the liner into any sewer laterals. In fact, sewer laterals are plugged by the liner and have to be reinstated (cut out) by a robot afterwards. At the end of the process the steam is released to atmosphere, it tends to have an odor. From my understanding the odor is styrene, a common industrial solvent.
Sometimes people with flaws in their plumbing report the odor is coming from the drain. This indicates they could potentially have a more serious issue: chronic exposure to sewer gases. I always recommend they have a plumber investigate. Other times the airborne plume is enough to cause very sensitive individuals to complain. There are many industry veterans who have been working with these materials for decades - if you were serious about studying the potential for harm, you would follow this group. They are the canary in the coal mine. Folks with only transient exposures are at very low likelihood of harm.
The American Society of Civil Engineers gave the sewer systems in this country a grade of D+. Just above failing. In reality, and I see this every day, our sewer systems are in need of immediate overhaul. CIPP is a keystone technology for renewal of these assets, because excavation is too dangerous(!) and expensive to be our primary mode of repair.
FYI - pipeline characterization and renewal is a growth industry with a lot of opportunity for AI. In my opinion, a lot of these processes could be automated.
Video below shows the CIPP process. https://www.youtube.com/watch?v=Swp81X4jSDk