I think there is essentially no downside and actually some slight upside, even more so if you mix them with regular basics. I actually wish Wizards would make more random downside so you had to make a decision but I think they’re totally fine to use and not worry about.

There are some random hate cards from Magic’s past (like Thermokarst) but you’re just never going to encounter that. Maybe if you’re playing casually and your friend wants to be annoying! But that’s probably it.

I know in Legacy for instance players will split their basics to make cards like Predict worse. Also any surgical extraction effect wouldn’t get all your basics if you split them, and any time you play something that lets you pick cards of different names it would let you get 2 forests etc (like Intuition). Playing all snow gets you cards like Skred for free.