Poland gets to plenty of immigrants. I doubt it’s the only former Warsaw Bloc nation that’s true of.

https://en.wikipedia.org/wiki/Ukrainians_in_Poland

In January 2016 the Embassy of Ukraine in Warsaw informed that the number of Ukrainian residents in Poland was half a million, and probably around one million in total. Ukrainian Ambassador to Poland, Andrii Deshchytsia, noted that Ukrainian professionals enjoy good reputation in Poland and in spite of their growing numbers Polish-Ukrainian relations remain very good.[13]

According to the NBP, 1.2 million Ukrainian citizens worked legally in Poland in 2016.[14] 1.7 million short-term work registrations were issued to them in 2017 (an eightfold increase compared to 2013).[4] Ukrainian workers stay in Poland on average 3-4 months.[15]

The number of permanent residence permits increased from 5,375 in 2010 to 33,624 (14 September 2018), while the number of temporary residence permits increased from 7,415 to 132,099 over the same time period.[3]

About 102,000 Ukrainian citizens received Karta Polaka,[16] of whom some 15,500 obtained permanent residence permits in the period from 2014 to March 2018.[17]

Is Ukraine that bad? Is it the looming Russian threat, the ethnic divide, a really bad economy, or an even worse corruption problem?

It's that you can get much better pay in Poland. Especially for lower skill jobs, higher skill jobs are not that different. It's easy to get legal employment and residence. The countries are close culturally and linguistically. And, of course, Poland is better developed -- materially and societally -- so the quality of life is better too. I think that at this point Ukraine can provide as many workers / migrants as Poland can absorb. And Poland can absorb a lot -- after its own big wave of emigration farther to the West.

It depends. If the labour market opens to Ukraine for example, it will have a positive net effect for sure. Why it doesn't open I don't know.

People said the same about Poland.