There may be the technical def for WYSIWYG that may differ from the conventional (consumer) understanding of it, but I think as it relates to most average joes it means the values of the indicators/knobs/sliders/whatever are always accurate to the setting of the parameter you’re using. So if the gain indicator/knob/slider/whatever is on 3 and then I flip to a scene/preset where the gain is on 5, but the physical indicator/knob/slider/whatever remains on 3, it is not WYSIWYG in the practical sense of the term. (From a consumer perspective)
WYSIWYG in software means something like this little text editor box on this website I'm using to type this post. It will show my post exactly like it will appear when I hit "Post reply." If I select the BB code version of it (brackets icon), it's no longer WYSIWYG but displays the tags and whatnot needed to do e.g formatting, bullet lists and whatnot. Only after posting can I see if my formatting is right.
A typical analog guitar pedal is WYSIWYG. Where its knobs point is what the values are.
The ME-90 is a lot like pedals with presets, where it's WYSIWYG with each parameter visible as long as you are just using the knobs to set the parameters. But the moment you change presets etc then the WYSIWYG aspect goes out of the window. It's a compromise, but despite obscuring the stored preset value, it still gives you really fast way to adjust something if needed.
Most modelers approach it from the other direction where they are better at showing stored preset parameters, but bad at showing and editing a lot of parameters at once. I can jump from pedal to pedal and do things to my fx way faster than I can get from my Axe-Fx 3 front panel or Axe-Edit to the right block and adjust parameters on the 3rd page and 2nd row of controls.
One of the reasons I've been asking for MIDI input improvements from Luminite Graviton's great dev is that it would allow me to build a "best of both worlds" setup - use the MIDI controller to craft control surfaces for showing stored preset values and secondary parameters, while still having most parameters right on the pedals to tweak.
I'm hoping the next generation of modelers from e.g Line6 or Fractal would put more emphasis on the editability, physical parameter control and display of parameters. I've got a big ass screen hooked up to my computer so I'd love to be able to pin different blocks to always show their parameters in Axe-Edit, or have a touchscreen capable of quickly picking the block I need and then using my finger or physical knobs to adjust stuff fast.