....over a narrow bus, and with controllers that have to make assumptions about power being yanked at any moment so they can't do proper SSD-like things that eMMC chips can do.
Narrow bus? I think current SD card standards can go way past 1GB per second (note: see edit below). It's an implementation detail.
In practice, these cheap boards will have a cheap eMMC soldered in that can't do these fancy things anyways. It's not going to be same quality chip like in flagship phones.
Once that cheap eMMC fails, the device is bricked.
Edit: SD card standards still can't go past 1 GB/s, max 985 MB/s. Either way, plenty for cheap SBCs.
SD cards as present on Raspberry Pi, even the Pi 4, don't go beyond ~60MB/s. Older Pis get around ~20MB/s. It's the worst bottleneck of the system. Where do you get that 1 GB/s figure from?
Bear in mind that SD Express is SD only in name and otherwise it is just an NVMe in SD form-factor. And how much UHS-II is still an SD is somewhat questionable.
odroid boards use a modular eMMC design. You can buy the size you want and swap them out if one fails. You can even see that in their picture (label M).
I don't think even flagship phones have "quality chip" inside. Phone OS's are still running without swap memory support to cope with poor quality of their onboard eMMC (which leads to OOM-killing "background" apps instead of just swapping memory out.)
In practice, these cheap boards will have a cheap eMMC soldered in that can't do these fancy things anyways. It's not going to be same quality chip like in flagship phones.
Once that cheap eMMC fails, the device is bricked.
Edit: SD card standards still can't go past 1 GB/s, max 985 MB/s. Either way, plenty for cheap SBCs.