I've been using first party isolation for quite a while on desktop and Android and the only websites I experienced issues with are the Atlassian login and the Trello GitHub powerup.
Everything else, including tons of single sign in/OAuth websites, work without any issue.
Yep, ditto Atlassian. There's a big thread of people complaining about it on their forums too.
Unfortunately of course it'll always be a minority, and won't affect whether even those people (have to) use Jira or whatever, so not really any incentive for Atlassian to fix it.
Everything else, including tons of single sign in/OAuth websites, work without any issue.