Being able to go in the Plane of Fire and talk to your enemy's boss requires more than "on your own term", whether you can cast a spell or find a portal.
Spells like Plane Shift requires you to have a plane-attuned tuning fork, and you need to know where the boss is pretty precisely. And the boss needs to exist in the first place, of course.
The dynamic is different, between "I have a spell for that" and "there is a portal for that", but both requires just as much DM-intervention.