Interesting. I thought this word is often used by incumbents to look down at the newcomer. English is not my first language so it's good to learn these subtleties.

When I think "upstart", I think of my grade 9 English teacher who once described a fairly naughty student as "you young whipper-snapper!".

To me it does come off as vaguely negative - in context, perhaps hinting at skepticism of whether the company will succeed in overtaking Uber. But if so, only mildly; the word definitely has its positive connotations too.

When startups were "cool" and people weren't so keen to their puffery and PR hacks or vaporware smoke tests, it was positive.

However, these days you almost certainly want to be known as a "business" by anyone who matters, and one with good reviews.