I do not know about Netbeans as I have never used it personally.
More specifically for GUI development, I believe NetBeans uses standard Swing components (and recently the Swing Application Framework which I think is going into Java 7), and I think Eclipse uses its own Swing framework.
No. There is no "eclipse" brand of Swing.
In eclipse, as long as the required classes are on the class path, you can use any third party libraries like JGoodies or JFreeChart or Quartz or whatever.
Does this mean that Netbeans is better/safer than Eclipse for GUI development, safer in the sense of standards compliance and language compatibility?
Not true.
In the end, the code is compiled by javac.exe which
will throw all kinds of errors if you do not follow the language specifications.
On the other hand, coding standards can and are not tied down to Java as java is a free flowing language.