At the heart of every MPPT controller is a PWM one. In simplistic terms, the capacitors store the power of the solar panel to be used later. As an example, the panel can produce 10A all the time. With a 50% duty cycle as an example, the FET switches on taking 10A from the panel and 10A from the capacitor bank. That makes 20A. When the FET is switched off, the 10A from the panel recharges the capacitor bank. The inductor also stores energy, but acts to change high voltage low current to low voltage higher current. Should the capacitor bank lose it's capacity to store a charge, the controller basically becomes a PWM controller. If the capacitor doesn't short out or explode, you may not even notice the change.
A capacitor rating is more than volts and capacitance. Like a resistor it has the ability to handle so much current. It is interesting that people put monster cables on a controller and inside the capacitors have #20 leads. A capacitor will have a temperature rating, say 85C. That does not mean the capacitor can run at that temp forever. Generally the life expectancy is 4,000 hours. Drop the temperature and it lasts longer. This is based on the liquid inside evaporating. No liquid, no capacitance.
A capacitor will have an internal resistance or ESR. Considerable current goes in and out. The weakest point is the attachment of the foil to the lead. Like a fuse it is easy to pop this open if a bad crimp is made. It is a chemical device so the purity of the components is critical. Tear apart a quality capacitor and the foil will have a nice even coating. Other capacitors can have a blotchy finish. Even quality manufacturers have had bad runs of capacitors when some minute impurity geys in. Even the worst capacitor maker can have a run that will last 20 years, but they can't do it consistently.
The market is driven by price. Delete a few capacitors or use lower cost ones and statistically you can get two or three years of life. These have to be shipped. The smaller the enclosure, the more you can get into a container. That doesn't bode well for heat sensitive devices. It takes a Herculean effort to get 40A on a circuit board, a custom super thick foil, added metal bars, and extra solder. It can be done for a short time and that is what they count on. Pretty hard to get a panels full amperage and that doesn't last for long periods during the day. And they count on someone having fewer panels than the stated max.
I wouldn't mind having one of these. The basic electrical design is sound. Some day these will be showing up on ebay broken after 5 years for $20. Then I can cut a hole int the face and add some extra capacitors. Check out this video demonstrating heating of capacitors and the advantage of parallel capacitors. IR imaging shows heating effects. Bear in mind these are at very small currents.
https://www.eevblog.com/fundamentals/ #742 There are other interesting ones for learning.