If you've worked with outsourcing firms and have had proposals for projects you might be presented with a ridiculously lengthy list of technological capabilities and systems experience. It's certainly right to wonder if they really do have strong expertise in all of those things.
Being Good at Everything
Could they actually be good with all those different technologies, sure, but I think it is fair to expect that the more likely answer would be "not quite". Recognizing the realities of acquiring clients and winning bids as an overseas outsourcer makes it easier to see why this might often be the case. You can't sell an engagement if you're not perceived as an experienced "expert" in the technology.
On some occasions when I've been presented with a portfolio from an overseas outsourcer, I didn't really recognize any website that they had complete responsibility for. I've seen where the outsourcing company was brought it to supplement labor from a domestic company, but, in that situation, it is almost impossible to know how much this outsourcing company did and how much of this talent is still with the firm.
Lost in Translation
Having this perspective, one can recognize that websites developed partly or in full by outsourced overseas contractors might pose unique challenges in terms of support, maintenance, and further feature enhancements. It might be difficult to track down the original developers to discuss documentation or the lack thereof, or to determine the intent of original code or if certain features were left incomplete.
Additionally, differences in language and cultural distinctiveness with respect to approaches to programming may become issues that need to be overcome. They aren't unconquerable though, and with the right team backing you up your organization can regain full control of your project and move the Drupal codebase forward to a more workable and documented state.