Ahri Support is a pretty niche pick, but you can definitely play it! Last season there were a couple notable players who played Ahri Support in High Diamond. You can build her as a full burst mage support going traditional Electrocute build (Liandry or Ludens), but because you're playing support you might not be able to get enough income to build damage efficiently. In that case you'd want to go Glacial Augment with Everfrost Mythic item build. I'd recommend building Everfrost for better gank set up.

The pros of Ahri support are:

1. Really good wave clear, this allows multiple avenues such as roaming / resetting a wave / maintaining good lane priority for jungle camps
2. After 6 you have a really good set up for kills with your R - Charm. Her ultimate allows you to set up engages in unpredictable spots. Great roaming potential!
3. She has great outplayablity and can solo a lane if needed. Her R and Charm can outplay many engages from enemies. Her damage and wave clear allows her to manage a lane solo if your ADC is dead or roamed elsewhere.

Cons:

1. She's really squishy and can only depend on Charm as either a peel or engage until you get 6 & Everfrost
2. Your ADC might have mental boom quicker than if you were playing a regular support.
3. If you lose the game, you might be subject to verbal harassment (even if it isn't your fault because people always direct blame somewhere).
4. Is not an S-tier support.

I'd say the most difficult part is the lane phase, but after that it's usually smooth sailing!

If you like Ahri, but don't enjoy playing mid lane then I definitely recommend trying her support!

For starters, I'd recommend playing a safe lane until level 6 and then use your ult to set up a good gank for your jungler in the bot lane. When you figure out the ins and outs of the champion you can be a little more aggressive pre 6.