Toxic executive management (Google the CEO to know what you're getting into)
Very high turnover (see number one)
Changing product direction. The focus of the product has changed many times over the recent past. Maybe this isn't a bad thing, but frequent change is telling.
Strategic hires and promotions are mostly internal from the managements circle (with some exceptions)
New logo velocity is diminishing (we have good logos, but ask yourself how many of them were acquired in the last year or two)
Layoffs. How many successful startups at this stage have had significant layoffs?