Ravenous Frenzy

I don't understand why this has a downside aspect to it when literally no other covenant ability in the game does.

This spell is quite complex about when it considers you as being idle. Simply moving or auto-attacking does not prevent the backfire. I have done extensive testing of this spell to track the idle duration with a WeakAura, and this is what I have found:

Valid as of Beta build 9.0.1.35755

You are not considered "idle" when any of the following are true:

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• You are casting or channeling and for 1.5 seconds³ after the cast completes or is interrupted.
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• You have successfully cast a spell in the last 1.5 seconds³. (This includes non-GCD spells)
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• Your GCD has ended in the last ~1.67 seconds.¹
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• You are affected by certain loss of control effects.²
• 
  

Note that spells that are off of the GCD (like Barkskin) do not give you another stack of the buff, but they do prolong your idle timer by 1.5s.

If this all sounds too complicated to keep track of on the fly, I agree with you, and that's why I made this WeakAura that tracks your idle timer: https://wago.io/65fcz54Ky

¹ For some reason, in addition to the time it takes for your GCD to end, you get an extra ~170ms after using an instant cast GCD spell before RF backfires. This means that after you cast an instant cast, GCD spell you have around 3 seconds (GCD + 1.5s + 0.17s) until the backfire will occur.
² Valid loss of control types include CHARM, CONFUSE, FEAR, and STUN. Notably, POSSESS effects (like [spell=605]) do not prevent you from being considered idle, nor do SILENCE effects.
³ Beta build 36165 changed the idle duration shown on the tooltip from 1.5s to 2s, but the actual duration wasn't changed. I guess they were just rounding ¯\_(ツ)_/¯

For all the upsides this might bring, having to nonstop cast as Feral for 20 seconds straight is ridiculous if you get unlucky with clearcasting and energy. This would mean I'd need to burn Ironbark and such just to not get punished?

Why take this over Convoke or Adaptive?