Open fields actually served Russia pretty well early in the war. The BTGs got a bad rep because they got stopped short of Kharkov, Sumy and Kiev, where the Ukrainian defence was thick and backed against large urban base, but the Russians advanced very well from the south (where Ukrainian defence was weaker). They did try to maneuver around Vugledar and got punished hard for it, but that again was pitted against a really solid Ukrainian defence. I think after Pokrovsk the weak defence and advantageous terrain might give a (potential) return to mobile warfare a good deal of success on the Russian side.