SE Oregon is 'high desert', an extension of Nevada. Lots of open space, some 'rims'.
Google Maps suggests as fastest (and shortest) US20 to US395 to NE California. That's all major 2 lane highways. There some hills has you cuts across Calif to Redding.
An alternative it gives takes OR140 over the Klamath Falls, and US97 to I5. 140 crosses a modest mountain range, with some grades, but nothing difficult.
The third is US95 into Nevada, and I80 to US395 (from the south) into Calif. That's longer, but not much slower.
Why are you asking about OR78? It's similar to other roads in the area, flat and mostly straight.
If you are interested in scenic driving off the main highways I could suggest some things in SE Oregon.