Powered risers use up to 5 A (AMD RX, nVidia Pascal cards), usually - about 3-3.5 A.
I prefer to avoid Molex and SATA connectors at all. The pins and terminals for 4 pin Molex are round and they do not fit 100%. It happens that 1-2 out of 4 Molex pins are not fully inserted into the terminals and the contact surface is barely sufficient. When the contact surface becomes tiny, the resistance increases and the contact heats, until it burns. Cheap cords and adapters from China are much more prone to this problem - the cable does not match the declared AWG rating (a cheap cable labeled as 18AWG turns to be 22AWG and holds only 1/2 of the expected current), the wire is steel instead of copper, the terminals are shallowly crimped, not soldered - a lot of weak points that might fry your equipment.
For ATX PSUs I recommend double-checking the fit of all Molex connectors.