Questions to ask.
MAINTENANCE
When was the last release? Is there active development? How quickly are security issues addressed? How long has it existed?
An abandoned project receives no security fixes, and you will eventually have to migrate away under pressure.
COMMUNITY
Are there people answering questions? Are there developers who know it? Is there documentation, and is it current?
REQUIREMENTS
PHP version and extensions Memory Whether it needs persistent processes Database requirements
Check these against your plan before committing.
EXTENSIBILITY
Can it do what you need without heavy modification? Is there an ecosystem of extensions, and are they maintained?
LICENSING
What does it cost, now and ongoing? Are extensions licensed separately?
EXIT
Can you export your content if you later move away? An application with no export is a trap.
THE PRACTICAL TEST
Install it on a subdomain and use it for a week before committing a project to it.