No it is not safe. Actually depends on the current draw through the wires. There were couple of incidents when I almost caught a fire.  On my first rig when I connected 3 risers with 3 sata/molex adapters, the temperature of wires was around 70C, luckily I have a infrared/thermal camera.  So never connect risers with adapters more then 2 in parallel. The other day the contact of one adapter burned out, and happily a rig just hanged. These adapters usually are cheap and wires gauge could be lower then needed.
One more example of what could possibly go wrong with the wires (don't related to the adapters): https://imgur.com/f1fkSCZ https://imgur.com/RZ8BH3R
This outlet was a first in line and nothing was plugged in it, the rig was connected to the last outlet in the line. The outlet was hot around 80C. So you can see someone, maybe the previous tenant, used wire connectors and wrong wire gauge to connect 15 amp outlet in 20amp line.