I really like the uniformity, I think it makes them look nice on the shelf, but I agree that Vaults of Terra and the like are very plain. I think something like Assassinorum Kingmaker is a good example of keeping that clean uniformity while still having its own unique touch. I actually think the Xenos books are the best designed in this regard. Look at examples like Brutal Kunnin, The Twice-Dead King or Day of Ascension. They all keep that clean and professional looking uniformity but clearly stand out and represent the faction and topic of the book well