I’ve used worse DIY cables and they were totally fine. And the connector I was splitting from wasn’t even SATA, it was a custom 4 PIN to sata connector. The server in question was a shady looking 1U rack-mountable custom Intel server.
The reason why the cables were fine was because I was using SSDs. As Dutch_Master said, the gauge and having a tight / not shorted-out connection is important. The DIY cables were cut from molex-sata adapters and soldered to a 4 PIN CPU adapter that coincidentally fit in the server where the power output came from (I did check the wires with a multimeter to make sure I connected the correct wires, so I wouldn’t fry my ssds).
Usually wires melt because people use thin wires for HDDs or GPUs, or overload a PSU that cannot deliver enough wattage on the 12v rail that they plug the splitter on. Wires melting doesn’t necessarily mean that a fire will start, but they can be the cause of the fire. I’ve seen lots of molten wires from overheating / too much power draw and they never started a fire. But I wouldn’t bet on them not starting a fire, I prefer to be cautious. Splitters have a use case, but you have to know when it’s safe to use them (just like with any power cord, or other things involving electricity).
ITT https://forum.level1techs.com/t/post-what-new-thing-you-acquired-recently/149881/5738 I posted this pic:
Cable management wasn’t the best, the PSU was an old-ish Seasonic. I didn’t use it for long, changed it with just a slightly newer Seasonic with black cables,
but I can’t find the newer picture / post
Found the replacement, I pulled my hair trying to find it. ZFS RAID Config for old disks - #21 by Biky
Now you can clearly see which cables go where. You can see that I used a molex to 2x sata, one for a SSD and one for a HDD. The rest of the cables were sata only, so I wasn’t splitting from the same cables, however, I didn’t check to see which rail the molex connector came from. But this configuration worked, because there wasn’t a lot of power being drawn through one cable. These were Seagate Constellation 2TB drives. I had 4 HHDs connected by 1 cable with 4 sata connectors (the ones on the bottom), 3 HDDs + the SSD connected to another cable that had 2 sata and 2 molex (only 1 used) and the last 4 HDDs in the 5.25" bays powered by another cable with 2 molex connectors. So 3 cables powering 4 drives each (well, technically one only had 3, because the SSD was consuming peanuts).
So, if you got only consumer ssds (enterprise ones tend to consume a lot more), it should be fine to split. If you want to split cables for HDDs, I suggest you don’t do more than 5 per single cable at most as a rule of thumb. If your PSU doesn’t have more cables, get one that does.
It’s a good idea if you have consumer SSDs or if you don’t have really power hungry enterprise SSDs.
As Dutch_Master said, this is false. All it matters is that you know when and how to use them.