Amtrak trains: Limited routes. Often delayed.
Greyghond bus: limited routes. Many stops. Transferring buses can be hours apart. Stops can be in the middle of the night. Stops can be in bad areas. There is more than one drop off stop in Chicago. One I wouldn’t want to be at even in daylight.
Megabus: limited routes. Bus stops at on the street, not at a station. Stop in Chicago is on the street, but in a well populated area, near the train stations.
I have taken the Megabus from Chicago to Indy and Columbus several times. Have had no problems.
My daughter’s friends have taken the train from Chicago to San Francisco four times. . One time there was a six hour delay. Other than that , no issues.
Note. Hiring a car refers to a car and driver, renting a car would be the car only.
From the few Brit’s I know that have visited, they have expressed shock at how big the USA is and how far apart towns and cities can be. I lived in Arizona for a few years. It is easy to drive for hours and not see anything but road.