Still not buying it. Playing a 8th note line in tune with wide interval jumps is much harder on an upright bass than violin/cello. The bass requires the same pinpoint accuracy and is even harder to hear the pitch. It literally takes more time to move you hand from point A to point B due to the gigantic scale.
In my view, rather than saying easier or harder, I'd prefer to say that intonation on violin and DB involve some similar issues, and some different issues. Violin has the issue that the notes are physically close together, and you're expected to play more of them. Check out a video of Itzhak Perlman. His fingers are wider than a semitone! Yet he plays perfectly in tune. Like you say, bass involves big shifts, and maintaining your physical endurance throughout a gig. Each is hard in its own way.
I've played DB (my main instrument right now), cello, and fretless electric bass. My kids play violin and cello, and I've observed countless kids progress through the stages of instruction on all of the stringed instruments. In my view it takes roughly equal amounts of time for kids to achieve good intonation on any instrument. None of the stringed instruments in the youth symphony stand out as being particularly harder than the others to play in tune. If anything, the wind instruments might be harder.
Now, those are the kids who got into the youth symphony, which means they played their auditions in tune. The ones who didn't achieve good intonation, didn't make it to that level, and probably quit.
But there are factors that are more important than the instrument itself. The kids who play well sound like they are obsessed with intonation. Somebody has taught them how to work on improving their intonation, and they are making the effort. They just won't allow themselves to play out of tune. So they have to figure out how to make their instrument work for them. That's the key to developing intonation.
There are doubtlessly many approaches to intonation, and here's mine: If you hear that you're playing out of tune, it's too late. That's good feedback, but it can't be your primary way of finding notes. It's about technique. Consistent intonation requires consistent technique. This requires an idea of what "correct" technique consists of, i.e., what standard you're aiming for. When I notice that I'm playing a particular interval or passage out of tune, I carefully analyze my technique and figure out what I'm doing wrong. I still remember enough from my classical lessons to have a standard against which to judge my playing. I'm doing that constantly, even throughout a gig. And for me, technique is not just for intonation, but is also a matter of survival, because bass is a physical instrument and bad technique can hurt you.