We work in a seriously developed Bitrix24, and on the client side we often see Jira and even Trello.
What's the difference?
In our opinion, Jira is more focused on development and is aimed at programmers, it is more difficult to manage complex projects here. Bitrix24, on the contrary, has no connection with version control systems by default, but it has complex tasks, visualization of terms in Gantt and so on.
We believe that the most correct thing is not to adapt production for the system, but to improve it for yourself and in this respect Bitrix24 with its source code wins significantly.
Say, our latest development is an advanced checklist.
Unlike regular checklists, now each checklist item has a person responsible, the items themselves are grouped into sections ("in work", "in testing", "on acceptance", "unassembled") with the ability to add their own groups. The checkpoints have end-to-end numbering and sharing of access rights (the tester cannot accept the work, but can give it for acceptance or revision, etc.).