Segments aren’t conceptually difficult, either, but definitely could be annoying, and certainly were, if you had to access data structures larger than 64 kB.
As to the differences:
- you had four segment registers that you could ‘point’ anywhere, allowing you to access four 64kB regions of memory without changing them (the equivalent of bank switching) (one always was used for accessing the instruction to run, one for accessing the stack, but you could use those for other purposes, too (Could, not SHould)
- segments can overlap. You could set DS and ES to the same value, for example.
Segments also can be moved at 16-byte granularity. If you wanted, you could have DS address address memory range 0x0000 ≤ x < 0xFFFF and SS address memory range 0x0010 ≤ x < 0x1000F.
Banking was one solution to the 1MB limit; was it extended or expanded mode? I can no longer remember, but one of those gave you a 64kb window somewhere above the 640kb limit in the address space not used by either video RAM or BIOS. That window could then be paged around the rest of memory.
https://en.wikipedia.org/wiki/Address_Windowing_Extensions
*: not actually 4GB, because that makes the kernel code harder to write and tends to lead to bad performance and lots of bugs. They could get 2GB max with the default settings and 3GB with a special configuration.
https://devblogs.microsoft.com/oldnewthing/20040812-00/?p=38...
I don't know about DMG/GBC/GBA games. Some very interesting stuff happened on those platforms (e.g. Game Boy Camera, and some game that lets you control a sewing machine in Japan?) and I bet a pure sliding window mapper exists.
The PC Engine/Turbografx-16 had platform support for mapping (specific CPU instructions did it) but it was 8 fixed windows in the CPUs 64K address space that pointed to 8K size offsets in the ROM I believe. SNES had a 24-bit address space and DMA to copy things to VRAM so not sure mappers were really on that platform.