I'm not sure if it's a problem as such, more just the way these FX units were designed back then. The SPX90 and subsequent SPX900 were initially used mostly in PAs and we didn't change programs via MIDI, doing FOH I'd use a SPX90 on the snare and leave it set. Program changes were not frequent and done handrolicly, usually between songs. I'd have a separate delay for vox and mute the return between songs. The issue with all time based FXs is what to do with what's still in the buffers (particularly delays or reverb) after you change programs. More modern units do this in different ways and also have become much more "digital". Also probably in response to different uses, e.g. MIDI controlled guitar racks weren't really a thing back then. Moreover not many guitarist could afford them.