There are many, many factors that go into epidemiological modeling. No two hotspots are exactly alike. There are generalizations and there are strong mitigating factors that influence each particular subset. Wuhan isn't exactly like Northern Italy, which isn't like NYC, yet the similarities are cause for concern. Throw in the fact that China may not be very forthcoming with the truth and it skews everything.
So look at history. 1918 flu to be exact. Tale of two cities, Philadelphia and St. Louis. Philly had a parade with over 200K in spite of the fact that the SF was raging among returning soldiers. Within 72 hours hospitals were full and a few days later 2600 were dead. St. L basically shut the city down. Death rate was 1/8th of Phillys.
You might say we have better medicine and infrastructure today. That's true, but we also have far more efficient travel - plane. car, rail. Larger populace, etc. Spanish Flu also had 3 waves. If we're lucky, we'll only have 2. Social distancing can buy time for medical personnel to treat patients and researchers to get out a vaccine and also find meds for those who are actively infected. Italy didn't get that time and NYC is running out of it.
