To answer your question: the 'glass canon' style of gameplay can work in many use-cases, but is limited to very controlled circumstances.
Plant is indeed a very powerful primary, and it might rank #2 after Mind. Why I would rank Plant after Mind:
Most of the power of Plant comes from Seeds of Confusion. So, any group that resists Seeds, like Nemesis, is going to heavily neuter Plant's effectiveness.
Plant lacks a way to handle multiple large groups of enemies that are positioned closed to each other, where Seeds is able to hit one group but not adjacent groups. Mind is able to put multiple groups to sleep for a long time, all without aggro, through using Mass Hypnosis.
Plant lacks a way to handle multiple AVs simultaneously. Mind can do that using Mesmerize.
It's for these reasons that Mind is able to solo Task Forces. Back in the day, there were videos of people soloing task forces with Mind, without Incarnate powers.
I don't know how much the game has changed, and I'm not familiar with some of the 'newer' primaries like Dark.
Here is how I would rank the primaries, again in terms of pure survivability. Please take this with more than a few grains of salt:
1. Mind
2. A close ranking amongst: Earth, Electric, Dark, Plant
3. Fire
4. Gravity
9001. Ice