It depends on the board. Some boards have balancing circuitry, some board can only detect overcurrent on individual wires, and some can only detect total overcurrent (through the shunt)

It is a combination of multiple issues. This is one of them

Bussing the connection on the card side means no balancing of currents between pins, like you said. Older cards dont do it per pin but do divide it into several input rails which are individually monitored for current / voltage differences and will disable the card if it exceeds a certain threshold.

Usually cards would use larger pins on the older 8 pin pcie power cables, which can handle more current and will make better contact in addition to being easier to seat fully without jamming and keep retained.

Additionally they tend to use multiple connectors (e.g. 2x8 pin connectors = 8 pins for +12V), with lower power allowed per pin for more headroom. This means higher current capability and less potential for failures

For example, a standard 9070xt with a 304W TDP may use 2x8 pin connectors. Larger pins, more pins and lower tdp, with current balancing/monitoring on board. It comes out to 3A per pin at full load when the pins are rated for something like 9A.

I think most of us have 12, but between 120 and 1200 I'd choose the latter because it would be more fun