Cell is essentially distributed memory cluster on single chip, because each SPU has it's own address space and cannot directly access main memory. I'm not sure about what the exact definition of AMP is, but it does not exactly match my feeling of what AMP should be. In this regard Wii seems more like AMP systems with two completely different CPUs (PPC and ARM) sharing what essentially amounts to be same address space (and in WiiU there are 3 PPC cores where one of them is slightly different than other two and cache coherency between them can only be described as broken).
There is no question of hardness of programming for Cell, but I think it's mostly about middleware support (probably because the platform is so different from PC and xbox360).
The real problem with the Cell was that each Cell SPE processor only has 256K of local memory. It has bulk DMA access to main memory, but that's more like I/O. 256K is too small for a video frame, a game level, or much else in a modern game. So everything has to be done on an assembly line basis, where data is pumped into a Cell processor, processed, and pumped out. Great for audio, terrible for everything else. In comparison, the main processor had access to 256MB of RAM.
If they'd had, say, 16MB per processor, it might have worked out. One CPU for collision detection and physics, one for NPC management and AI, etc. But giving each SPE processor only 0.1% of the total memory space was too constraining.
There is no question of hardness of programming for Cell, but I think it's mostly about middleware support (probably because the platform is so different from PC and xbox360).