That's a little like asking is medium well looked down on by steak eaters.
What you should be considering is making it look as good as it can look.
If you're talking about what brings better prices in galleries, mixed media over the purity of like, say a watercolor, or oil. . . or oil over acryllic. . . I don't think it matters. Permanence of the materials may be a factor, like if part of it's going to crack and fall off the canvas. . . that's possibly an issue.
And watercolor society people have their pet purity guidelines like breeding pedigree poodles or something. They would generally look down on using opaque watercolors (gouache) along with straight watercolors. And it may affect prices in those circles.
As to the reason mixed media could be a drag would be if they want you to make changes.
Personally, I like gutsy paintings and drawings a lot. And mixed media works for me, different styles in a single piece, all that is pretty cool if the art sings.
If the art is good, and people with money like it enough to pay your price, there's part of your answer anyway.
If it matters to the people you want to impress, it matters. There are no absolutes. Paint it the best way you can. See if your client likes it.
Specifically for digital stuff, facile filter effects may make an image more common, and that an unskilled person could do it. I think that may lower the value, especially if it looks like it could have been achieved by clicking a single filter. Boom done. Don't expect that will impress many.
But if you use filter effects with artistry, then that could be very appealing.
It's a difficult question to answer. When one deals with art, snobbery can rear its ugly head at the least provocation.
Hope this at least covers some of what you're asking.
"Not a bit is wasted and the best is yet to come. . ." -- remembered from a dream