What is the reasoning for this? I think JVM still has problem handling real large heaps, the GC stop will be unbearable..Does thing change recently?
There is simply a gap: A 30 GB heap (with 4 bytes pointers) may be similar in capacity to a 40 GB heap (with 8 bytes pointers).
Guides and documentation (elastisearch) are simplifying this as "java can't do more than 30 GB heap", which is factually incorrect. You can do a 60GB if you want.
That's not true, dealing with large heaps is problematic in any garbage collected language because due to fragmentation you end up in stop-the-world full GC phases. Which for a big heap (i.e. bigger than 4 GB!), you can end up with latencies measured in seconds or even minutes.
The article mentions the CMS GC for Java, which has this problem. The G1 GC, introduced in Java 7 I think, is much better, however from what I've read it too isn't immune to full GCs. Of course for Java you also have the Azul "pauseless GC", which is supposed to never do this, but it is a commercial solution. And the article also mentioned another GC currently being contributed by Red Hat, which is good to hear.
Sure, if you go from 300MB to 30GB, the GC might behave differently and maybe some applications can't take it.