Ironically this might be the most PRO-ukranian measure EVER. Let me explain why. Baltic countries (e.g. Estonia) after dissolution of USSR had a luxury to base their citizenship on more than just a mere residency, this way they managed to make most of their russian population non-citizens. Ukraine did NOT have such a luxury, citizenship was granted based on mere residency.
Now Ukraine is given a chance to make the same trick which was performed by Baltic countries in 1991: make Russians non-citizens. Any ukranian citizen who takes Putin's offer and becomes Russian citizen will trigger Article 19 of the Citizenship Law of Ukraine which specifies loss of Ukranian citizenship. You see, all those people can be legally stripped of their Ukranian citizenship. At the very least they lose their voting rights in Ukraine. They also lose their right of abode in Ukraine and can be deported to Russia.