Comparision elgg & others
Elgg is more reliable, having lots of plugin functionalities. They are providing API plugins to add functions / modules into it. i feel that is their greatest value added thing.
Other than that, elgg is fine, but compare with some other opensource platform they dont have more services.
If you are looking for a readymade services, elgg is not a good choice. But if you looking for a platform to build on, elgg is really good choice.
Follow the guide, how to install elgg
Customization of elgg platform
Get the expert guy in elgg, do the customization whatever you want. as i told you in the start, elgg is having lots of features, lots of customization options. You can develop or add any plugins, applications into elgg platform.
Issues with elgg
i. elgg is bit slow
ii. elgg is not good for beginners