>I own an UltraProteus with almost 500 filters and an extensive mod matrix. Pretty sure it can mangle a sample quite a lot.
Which is neither here, nor there. I didn't say you can't change the sample "quite a lot", I said you can change it much less than if it was as a synthesis patch on a synth.
Any fx or filter available to the sample is also available to the synth. The reverse is not true.
As a simple example for an extremely basic thing you can't do: you can't change the sample's inherent attack. The ASDR envelope on the sampler works on top of the raw sample's attack, so you can't, for example, change a slow attack to a fast one.
Much much less.