Think of any task that has ever caused your laptop fans to spin up, and cross that task off of what is possible on a phone-as-desktop list. No high definition video calls. No 1080p gaming. No fully functional IDEs (intellisense and interactive linting/compiling is computationally expensive). No high definition movies. Video/batch photo conversions are probably out, though may be possible at 1/10th speed.

The most successful tablet-laptop attempt thus far has active cooling simply so they don't have to throttle back performance.

It's possible that more energy efficient processors will come about and prove me wrong, but they've been working on the heat dissipation problem for decades now (yes, even in desktops/servers).