I've used three different engagement software, so I know partly it's because of my previous experience/expectations and how I use my campaigns. That said, the least intuitive part of adding people to a campaign is after uploading a csv file or adding a single contact.

The workflow is to click on a campaign, then click on "add prospects", which brings me to a list of all the prospects that are not in the campaign. That means if I want to add new prospects I have to click on "add new prospects", which brings up a modal to upload a file. Since sometimes I want to just add one prospect. Firstly, it was not intuitive where I could add just one prospect. I finally figured out you could do so from the dropdown menu. But then, you have to manually add them into the campaign, which is a bit much given that I arrived at the "add prospects" page from a specific campaign. And doing so does take too many clicks, you have to find them from the list of contacts, which can be many if there are several people on your team, check them off and then click on "add to campaign" and then finally add them. These last clicks have led to miss adding prospects to campaigns since I expected they would be added automatically.

If I'm adding one prospect to one campaign, I'd ideally want to do that from the campaign and/or not have to click around so much.