Uncontrolled bipolar has some pretty heavy side-effects too. One of them being death. numbers vary depending on studies but about 17% of people with untreated bipolar disorder will successfully kill themselves. That number drops to 8% with medications. Something close to 50% will try and that can leave people with damage to liver and kidneys.
It will be important to get blood tests done at the six month mark, and yearly and use that information to inform yourself of treatment decisions.
The side effects are real and they are scary. Bipolar medication is horribly understudied for long term effects (all medications not just bipolar). So the honest answer is they don't know what the long term prognosis is going to be.
I take medications and I do so knowing the benefits and risks. I have studied what I take extensively. For me the side effects of bipolar are far worse than the side effects of the medications. For me I regained a life taking meds and I appreciate that life.
Knowing what I do though, if I had cyclothymia or was just mildly depressed at baseline I wouldn't take them. But I was nutbar crazy so we needed to use the good stuff.
It doesn't mean you're going to get all the side effects just because it's associated with them.