The Hiawatha has departed ontime to the minute from either CHI or MKE on every trip I have taken with no noticable delay at the destination. That said the train does not stop at Milwaukee airport in the sense you would think. amtrakdelays is showing an average delay of 6 minutes, albeit with limited data for 339.

The station is actually about a mile away and requires a shuttle bus ride to the terminal, figure 10-15 minutes to allow for possible waiting time. I think the train can be up to 25 minutes late and you can still make it, a tight margin of error but likely doable.