Backplane capacitors are most likely irrelevant. They are there to provide some high-ESR bulk capacitance. Most ripple current flows through the caps near the SMPS switching inputs and outputs, or near the fast switching loads (CPU, GPU).
Now, the caps at the power supply outputs are important, they carry the ripple of the PSU switch mode converter (and are also located near to the components that heat up in the PSU, since they are needed exactly there). Similarly, caps at the input of the local buck regulators (near CPU, for example) are exposed to high ripple currents of the said converters - and the heat, too.
Backplane caps are further away and hence, carry much less ripple -> less internal heating.
They are also located further away from heating components -> less external heating.
Also, they are less critical to the functionality -> even if they "go bad" (the ESR rises), it's probably not a big deal.
It's normal design to use lower-cost elcaps in places like this. Actually high ESR is sometimes desired.
Blindly looking at all alu elcaps to asses the design quality does not work. You need to know where to look at.
OTOH, if the local bypassing at the PSU output / local buck input is lacking, then these caps might carry significant ripple. It's impossible to say for sure.