The currently supported version of Unity3D is 5.3; any previous version from the 5.0 line should work, however is officially unsupported. Any version from 5.3 onwards will be supported, except where specifically noted.
We will be commencing the 5.4 update shortly; we often delay the latest release update for 2-3 months to allow Unity to release bugfixes to the editor. Often we find new release versions of Unity contain many crash and loss-of-work inducing bugs; along with bugs that users may experience. If you are using another version of Unity in another project, you should note that it is possible to install multiple versions of Unity side by side.
The following resources may be useful for those who want to learn Unity itself while learning how to create using the Space platform.
Most Unity features are supported. Generally speaking, what you see is what you get when utilising the editor pack. Some upload modes may impose restrictions on specific Unity features, but generally should all operate as desired and intended.
The exception to this is custom scripting behaviours; we do not allow these to be distributed with scenes or content (or more specifically, simply do not support this feature) as Unity's scripting environment is dangerous for end users (allowing things such as File System access). Content utilising these will upload, however the behaviours will be removed from Game Objects by the processor prior to export.
These extensions to Unity have been officially supported by us, and can be used when creating your content; most of these extensions can be found in the Unity Asset Store. Please note - some parts of these extensions may be explicitly excluded. If you wish to use a extension that is not in this list, please contact us via the Support Portal, with 'Regarding' set to 'Creator Program'. Due to code size limitations (namely limiting download sizes for end users of player files, particularly WebGL), not every extension can be supported.
Policy: If we depreciate an extension, content will no longer be allowed to be submitted using this extension. Existing content will continue to work as long as it is possible to support. This should be infrequent, but may happen due to engine incompatibility with upgrades. Sometimes, we may choose to replace one supported extension with another (we will likely depreciate USequencer once the Unity built-in Director Sequencer is released in 2017, we will however keep content developed with USequencer working until the vast majority of users have transitioned however.)