I was originally thinking of sticking with my 13 units in year 12 so I would be 'safe'. Until I got so sick of having no extra time to do things or have time for rest. I was never bad at biology, I was ranked like 13/105 students or something like that.. but I found it boring AF, I swear.. like every lesson I would be on the table sleeping or so tired, and the period afterwards (not bio) I would be completely fine.
So after half yearlies (I know, i kept it for so long lmao and I dropped before the interesting part - the human body), I decided to drop biology. However, I found that I didn't have more time to 'study', nor a less work load because I never had homework for biology. Rather, I had more time for myself, to hang with my friends and have fun lolol - most free periods were unproductive for me, I usually go to the shops or just chat to my friends but it's a good way to get away from all the work. Even if you tried doing work at my school, you won't be able to concentrate due to the noise in both the library and the senior study (not to mention stink).
I don't really regret dropping bio at all. I'm rather fine with my 11 units - this is more safe than 10 though. But at the end of the day, it doesn't come down to whether you think its 'safer' to keep something, rather, it's what you enjoy doing and that'll affect your overall mood in school. Most of my friends are doing 10 units, I had a friend on 15 but she dropped down to 11 nearing the end of year 12 haha. I heard in schools like sydney girls they encourage you to do 10 units or something... not sure if it's true though.