In general, sliders should never be created with Images with text within them as the images will scale differently on different devices.
But here are some best practices when creating Carousels within the Site Builder.
Static & Inner Static
Sliders on Sites with set widths (i.e. "Static" and "Inner Static"), we do have logic to set the Aspect ratio to maintain the similar view for each breakboint. This should allow of text-based background images.
Sliders of 100% browser width (Fluid) should never have text within the background images. Since the User's browser width is unknown and the image always scales to the width of the browser. No Automatic sizing is done to maintain the aspect ratio. It only uses the Browser width and a set height and fills the carousel area with the Image.
"Slider" modules should also not use any text within the background images (similiar the to the Theme "Fluid" option), as these modules are within flexible column widths which change based on the breakpoints.