When you opt to use a multi-platform development kit, you have to utilize a new set of system calls in your code, link to new libraries, and in some cases utilize a run-time engine. Unless you plan to maintain the kit yourself (and have the right to do so,) you are dependant on other people for enhancements, bug fixes and insuring the kit will work for future versions of the target operating systems.
Trolltech, developer of the QT multi-platform SDK, actually open-sources their code under GPL. If you use the kit for commercial purposes, then you pay Trolltech a licensing fee, otherwise their kit is free to use. If Trolltech ever goes out of business, people can continue to enhance the kit, because they have the source code.
I've been in a situation where a company had based a critical piece of their software infrastructure on proprietary middleware. The middleware vendor was then sold to another company, at which point the brilliant founder, who was also the lead developer, left the company. From that point on, no updates were released for the product. The source code wasn't available. Worse, not having updates prevented the user from upgrading to new hardware and operating system versions. And they couldn't just throw the system away, their business depended on it. Overall, a bad situation to be in.