A downside to surge brakes is you lose the main way to reduce/stop trailer sway after it has began. I do see many advantages too.

We had a flatbed Trailer with hydraulic surge brakes that came with a 6-wheel ATV at work. It towed and stopped fine for the short time we used it. After a few uses we swapped the trailer to another department for a smaller trailer needed for another purpose, and later bought a covered trailer for the 6-wheeler.

The flatbed would not back up AT ALL unless you wired the Tow vehicle's backup light power to activate a bypass soleniod. The trailer had a flat 5-pin connector for this, and we plugged it into our regular flat 4 light socket plus added a 1-pin socket for the backup pin. Once this was done, driving and backing was fully automatic. I don't recall there being any way to get out and manually disable the brakes for backing this system without connecting this wire.

The brakes worked any time the trailer was pushing against the tow vehicle - going down hill, slowing or stopping.

One safety item I saw on another forum, and hadn't though about, is that if the tow vehicle enters a slick surface, and does not slow when brakes are applied, the trailer brakes will not apply. With an electric controller the trailer brakes could be manually activated to help.