I think the statement regen sucks is a little over the top.
Naturally.
I don't really grasp why it would suck when I use absolutely no mechanical brakes at all and can stop as fast as I want without wearing down brake pads, while charging the pack simultaneously.
I mean regen sucks compared to most people's expectations of it. It's not a magical energy device. The energy lost in braking is miniscule and won't impact range.
In people's heads, when they hear and think of Regen they're thinking of like, 40% increase to their range. If you told them it was more like 4% no one would care. If you told someone that this amazing new system improved their gas mileage by 1 mile/gallon no one would care or think anything extra of it.
You can do magnetic braking on DC without doing regen.
On Forklifts this is called "plug" braking, where you use a double-throw-double-pole contactor to flip either the field or the armature (and, for some controllers, it matters which one gets reversed, protection diodes are the issue I think). This is functionally putting the car in reverse, so you're applying energy in order to slow the car down, making the motor burn double the energy it normally would IIRC.
There is also resistive breaking, where the field is toggled out with a massive resistor I think. That doesn't consume extra power, but it still dissipates it as heat.
Either of those options magnetically slows the vehicle without using the brakes, but neither of them actually adds energy back into the pack. Which is probably fine for most people, since it's a mild benefit compared to the ones you listed.
And they had issues with it in the 1980’s when someone tried it in a specific use case on poorly designed hardware.
I don't even really understand, at it's most basic, what is happening during DC regen. How do the wires change, how is the energy recovered, why was it considered impossible before but not now?
I probably couldn't understand the whole circuit, but, just a layman's explanation of the basics would help me.