You should almost always take exhaust on any. Even on Zyra which I play 6 out of 10 games because of her support carry potential, I only consider ignite againts soraka lanes(she is highly banned in high elo games, but sometimes she slips thru).

The reason is simple, you already have someone with ignite on your team(your mid), and nobody else will take exhaust.

In a usual game, especially at low elo, theres gonna be that one lane who will feed hard, so you will have to deal with that 11-3-2 Yasuo once your lane phase ends.

Ignite will simply become useless late game(every decent comp will have 1 deleter, who without reduction will simply delete one of your team members (hint:your carry), and reducing hes damage is the only way in a tf with zyra. The 13-4-10 zed just gonna ulti and all you can do is watch your carry die while he teleports back.

With Zyra you have an insane amount of kill pressure even without ignite, you should win your lane without that 120 true damage.

Also if you solo que and your adc is braindead(I'm sorry) and constantly gets pulled by blitz, then ignite won't save him, but an exhaust could.)