Enabling multiboot is something that only should need to be done once, assuming it is done correctly. It is persisted in the uboot environment stored on emmc.
Having said that, I have experienced cases where for some reason on some boxes the uboot environment gets reset to the default and multiboot does need to get re-enabled, but that is a rare occurrence, nothing I have ever seen happening on every boot.
Your assumption is incorrect. The 'multiboot' changes the uboot environment stored on the emmc, even if you are trying to run something on sd. The is the whole point of 'enabling multiboot' without the changes to the base uboot environment the board doesn't know how to boot from the sd card. Those changes to the base uboot environment are different across different distributions and therefore the requirement to restore back to a known base with the original android firmware.
Amlogic has quite good performance/price ratio: their low-end S905X3/X4 are very good chips for the price, and quite updated too (Cortex A55). Rockchip and Allwinner have nothing comparable yet for the price. Allwinner is far behind. Rockchip recently introduced the long-awaited RK356x series which at least is on par with raw performance to S905X3 and has a nice set of features, but the price is clearly higher and support is still going on. RK3328 is not as good as S905X3, either from CPU and GPU sides, but the RK3399 is still quite good SoC. Amlogic has the best chip on paper with S922 (and similar ones), but in the past they did some double-cross with frequencies and temperatures so people is reasonably skeptic on the real performances.
Despite lagging behind, Allwinner chips are at least very cheap with decent raw performance (H6 at least), but the company is a bit silly.
About linux and community support, Amlogic is the worst one by far, being quite obscure about their hardware and generally not very supportive of opensource.
Rockchip is the best one, a lot of their drivers are production ready in the mainline kernel. Recently although I saw quite a stop in their "proprietary" kernel and u-boot public forks. I don't know why they stopped, but I hope it's just a temporary reorganization: the effort they did in supporting opensource was very appreciated by the community and mainline kernel is very advanced on supporting their chips and peripherals.
Allwinner in the past was a total wreck, a lot of work by community has been done to reverse engineering things with excellent results and I think they now opened a bit more publishing especially documentation.
In the near future I plan to publish the Linux images for s812 (they are being tested). TV set-top box on the basis of the s812 chip is very close with s802 (they all use the same kernel). So I hope that the images s812 will also work on s802.
