Dia is in this list of tools surely the grandpa of all tools. It was started 1998. So the tool was developed not with the mind to develop a free UML tool. It was developed to enable the users to create any form and type of a diagram.
So Dia cannot be considered to be a UML tool like some persons are telling. It is more something like Visio or a more structured Presentation tool to show a diagram of something more complex.
Sadly the last published version is from end of 2014. Also the commits are steadily degrading, so that we can assume the project is as of Jan 2019 dead.
The following analysis is based on the requirements discussed in Free UML tool for developers. If you want to now more about the following requirements, read the named article first.
Project health status
Dia development seems to be dead, since years no active commits are shown
The community for maintaining Dia seems to be inactive, since no active commits are shown and thus no active maintainers are visible
Tool implementation language
Supported operating systems
Forward engineering (code generation)
Reverse engineer code
UML 2 support
Integration in Eclipse
Integration in IntelliJ
If you want to have a full UML tool, this tool is surely not the right choice. It is pretty good if you want to create quick diagrams and you do not have such a tool like Visio. Anyway in summary we cannot recommend it for usage on development projects
Want to learn more about UML tooling?
Read our article series about UML tools which start from here: Free UML tool for developers