1) 10-hour time difference between West Coast US and, say, Kyiv is pretty big. It's easier on EST people, but on the US side you're going to have to plan on jamming your calendars full of meetings between 7:30-10:30am.
2) Because you have fewer "business hours" between two, you need product, project, and engineering management to operate in a fairly well-oiled manner with more stuff spelled-out up-front, otherwise your team across the pond is stuck until the next day for some answer. Also, if your org even attempts to adopt "agile" this communication breakdown will murder your velocity; you're better off doing Waterfall.
3) They cannot be on-call to troubleshoot product issues during US business hours. This makes the business-side uneasy
4) Higher churn-rate than domestic engineers. Mostly over salary; this falls squarely at the feet of the "middleman" who undercuts their engineers. We attempt to ameliorate this by giving them feedback on employees who they should focus on retaining but IME they just view their talent as widgets rather than craftsmen. I've seen several good devs leave over money we'd have gladly paid them but their contracting org refused.
5) Speaking of higher churn rate: Onboarding can be more painful, again because of the TZ difference as well as certain cultural issues.
None of these are deal-breakers. But it's easy to understand why a company might be willing to pay 50-100% more to avoid these concerns.