When we visited it in Sept, we were never able to buy tickets online in advanced and were forced to show up and que in the street, it took about 50 minutes to get in. You can walk inside and look at the monitors and see how many tickets are available for each time slot ahead of you, count the people in line and know about when you will get in.
The 2 complaints I have are
1) We knew in advance that this attraction has little to do with Schindler and is all about the Krakow experience during WWII. I have no idea why they continue with the way it is marketed. Every advertisement of it uses a photo of the one room of things associated with Schindler's factory to advertise it, which makes up about 0.000001% of what is on display inside. Seems really dumb as the subject matter is worthy on its own right and done well enough that it doesn't need to be shilled as something it isn't. There are so many people who are wandering around just trying to find something to do with Schindler's factory and not interested in the excellent presentation that the museum holds, seems a waste to do this this way in 2025.
2) Don't make people show up and hope to get tickets, all those tickets being held back should be released on the website
While a very good presentation of Krakow during WWII, the place just seems like it's run very backwards