"Bad UX" can't be generalized that easily for a simple visual effect, that's highly dependent on the integration.

A good integration would:

- consider how heavily to use the effect to not overload the ui

- query for feature support to avoid adding all additional decorations if blur isn't available

- query for display & video features to avoid rendering the effect on devices that likely don't have a capable gpu

- query for user preferences and serve solid colors with good contrasts to users who might struggle with the busier background

- limit the extent of the effects and shadows depending on available space

- tweak the blurring and opacities to ensure good readability in the specific UI, regarding how complex or contrasted the background under blurred areas will be

- ensure blurs and shadows scale adaptively with the typography to ensure consistent visuals

UX is by definition the design of how a user experiences the complete app and interaction, it's not made or broken by individual stylistic choices