They're good for marketing and a manager because there's a number of nice adjectives about them. "modern", "minimalistic" etc. They're bad for users - because they all look the same and lack color as well as shading.
I get what the point is (i think). Symbols like we see that indicate male vs female gendered bathrooms, the 'Handicap parking' and others allow everybody (and i hope all/most cultures) to instantly recognize what is being depicted. I would be hard pressed to guess what half of these are trying to represent.
That we recognize a gendered stick figure on a door to mean "this is a toilet" is not less arbitrary than the "WC" signs you see in Europe, which at least have some meaning ("water closet", olde timey for "toilet")
Does it being colourful or having shadows impact the representation?
I don't think colour or shading has any impact on the icon. I think proper accessibility by labelling said icon is 100% more important then anything else, unless it's a universally understood symbol (such as floppy disc for saving).
A similar disaster were the new gimp tool icons:
https://www.gimp.org/release-notes/images/2.10-icon-themes.p...
They actually brought the old ones back as an option.
I definitely think it should be an option though for people that find the colored icons easier to find when searching.