Why is an Active-Active configuration not recommended by F5?

You cannot use APM module in Active-Active. If you don't care about APM, there's nothing wrong with Active-Active if you know what you're doing.

The main question is, what do you expect to get out of Active-Active?

Personally, I see Active-Active as a great temporary solution to scale for extra capacity during annual peak-activity events. With the Active-Active setup, you can also provide the 'means of survival' until next budget year. For instance, if you run low on capacity, but the budget is already locked in for the current business year, and you cannot immediately purchase more powerful appliances.

In any case, it should not be the default option because of the risk of losing high-availability. AMG also mentioned that. I'll just add that constant CPU usage over 80% is already pretty bad, you want to aim for 60% or less usage. If you see 80% CPU usage in 5-minute average graphs, you can be sure there are a plenty of short bursts (that last for a few seconds) where usage is capped at 100%.