Hi High -
The appropriateness of a route can vary depending on your age, level of experience, and time of year.
I'm guessing that you are in your 60s, live in the East, and are planning on riding east to west.
I've ridden cross-country a half-dozen times - all on routes I planned - but have done most of the TransAm.
There's nothing wrong and much right about planning your own route - but not on Google, necessarily.
Early May can be chilly and wet in Upstate NY and western New England - it was this year for sure.
Large metro areas are, for the most part, tough to ride through - college towns have all cyclist services.
Once you get west of the Missouri, there are some major WTF sections - the problem with Google maps.
South Dakota - lots of remote dirt roads. Wyoming - dirt road west of Sheridan, nonexistent road in Yellowstone.
But the biggie - the biggie is in southern Idaho and northern Nevada where you are on two-tracks.
Now, if you have experience on remote BLM land with no water, no cell coverage, and no markings -
Then yes, you might be able to do it. Otherwise, it is classic Google crap.
The I-80 section in Nevada sucks - legal to ride Interstates in the West, but why?
Then it has you on remote dirt roads in the Carson Sink - brutal heat and sand - think Donner Party.
Finally, US 50 over the Sierras has heavy traffic and is a poor choice for cyclists.
Nearly every state has county road maps and traffic count information on their websites.
Google is not your friend. Especially in the West where it is notoriously bad.
You can make your own route, but you will need to more footwork.