There aren't many sources I really trust, but here are two.

Here's what the Breast Cancer Fund has to say about bioidentical hormones:

There are very few studies, and those that exist have somewhat contradictory findings regarding impact on breast cancer. On the other hand, differently aged women, different compounds of hormones, different duration of using hormone supplements...all make comparisons difficult.

We are concerned that even taking bioidentical hormones extends the lifetime exposure of women to estrogens and progesterone, given the extensive data linking duration of hormonal exposure with risk of breast cancer. We also recognize that that there are differences in biological potency of various component estrogens, and that pharmacies and/or companies differ in their recipes for compounding the bioidentical hormone components.

In short, we support inclusion of bioidentical hormones in ongoing studies of efficacy and health outcomes (positive and negative), especially for post-menopausal women using these compounds to address the symptoms of menopause.
The National Women's Health Network is less equivocal:

All hormone products, bio-identical or not, are manufactured in a lab. While some are synthesized from “natural” sources (horse urine, plants, etc.), all go through extensive treatments in the lab to become hormone products. Some are synthesized to be exact copies of the body’s hormones (e.g., estradiol, estrone and progesterone); others are made into forms that are similar, such as progestins and conjugated equine estrogens. Unlike herbs, there’s no place in nature women can go to harvest hormones for home use. An interesting aspect to the natural vs. synthetic argument is the little known fact that many pharmacies that promote bio-identical hormones as natural alternatives actually buy the raw ingredients from pharmaceutical companies!

Are bio-identical hormones safer than other forms of HT? No. Are bio-identical hormones more effective than traditional HT? No, again.