The SNES has a 16 palette limit. 8 for sprites, 8 for backgrounds. The PS1 has a substantially greater amount.
The SNES has a far more limited VRAM. Not as many sprite or background tiles can be loaded at once, giving duller and less detailed scroll patterns. I can go into greater detail here, in how Rockman & Forte's player sprites are handled in a completely different way than Megaman 8.
The SNES was limited to 4 background scrolls (layers). PS1 games do not have that limitation for the most part. Layers are 'logical' and not 'physical'. They coded the game to display textures in a grid pattern, thus creating 'layers' but the hardware itself does not have 'layers' natively. Most 2D games have to do this, since current hardware typically doesn't support actual layers or have crippled and or limited use.
Case in point: The DS actually has layers, but none of the Castlevania games use them, and I don't believe ZX and ZXA do either. The reason is, you can get far better effects by simulating layers instead. The DS doesn't offer a full range of effects that you can get using the 3D render routines instead.
Similar example: Computer harddrives. You can section a physical single harddrive into many partitions. Windows will treat these partitions as if they were each a separate drive. Partitions are 'logical' drives, but they all exist on the same 'physical' drive.
In conclusion, while there are obvious hardware limitations that do effect things in this instance, there is also some due responsibility of the developers to make the most of what they have. Some portions of Rockman & Forte did well, and others did not. It's not a bad game, but you do wonder when you look at games such as Seiken Densetsu 3, or Tales of Phantasia, how much better it could have been visually.
I hope that clears things up a little. I can give more detailed info on any of the sections if you are curious or confused.