Things to consider…
1 Does it sync?
Is there an iOS or iPadOS version of the app. How do they sync? Some apps use their own servers to sync, while others use iCloud. Consider if you even need a separate mobile version of the app.
2 Single- or multi-user?
If you want to use the productivity app as part of a team, you’ll need features that enable you to share and delegate tasks.
3 Does it work with other apps?
Some of the apps here enable you, via share sheets, to add tasks from other apps, like email clients. That can be useful if you regularly create tasks from email messages or calendar events.
4 How does it show upcoming tasks?
A good task manager will highlight tasks that are due on the current day or that are overdue.