I will be the first to admit I don't know anything about the art or science of war. I do know that Ukraine is in an unenviable position. It is fully encircled. Most people are aware of the buildup of troops along the borders between Ukraine and Russia, Belarus, Donbas, and Crimea, which blocks off three sides. Most people are unfamiliar with the final side bordered by Transnistria. On most maps, you only see Moldova along the western side. Transnistria is a breakaway "province" of Moldova and is not recognized as a separate country by the UN. It is similar to Donbas and Crimea. It broke away with the active participation of Russia.
If Ukraine were only facing one front along the Russian border, I would feel more confident. Ukraine would have a high ground advantage. Ukraine soldier entrenchment gives them a strategic advantage, despite Russia having more significant numbers. As it is, Russia could attack from any side with a much smaller force and cut off sections of Ukraine from each other, destroying supply lines.