Only a relatively small percentage of California doctors accept Medi-Cal currently, because the reimbursement rates are abysmally low. There is, for example, only one cardiologist in the entire greater Oakland area (SF East Bay) that accepts Medi-Cal. One doctor in an area with around a half-million people. There are a lot of GP's that take Medi-Cal, but specialists are hard to come by (and the majority of doctors are specialists).
The states insurance commissioner and the head of CA-HHS have already made statements advising people to check out the provider networks of their plans before choosing one. As an example, the list of providers offered in a "regular" Blue Shield plan is going to be substantially larger than the provider list offered in a CoveredCA Blue Shield plan. When the new plans were put together, reimbursement rates for the cheapest "bronze" plans were essentially cut to Medi-Cal levels by most insurance companies, and a lot of doctors who take "private" Blue Shield plans opted out. This has fundamentally created a two-tier system for people using Blue Shield. If you have a private Blue Shield policy, you can access their entire network. If you have a CoveredCA Blue Shield policy, you will only have access to a portion of their network. The same services are available, but with far fewer doctors.
My own doctors office sent me a letter two months ago about this. They made the decision that a couple of the doctors in their practice would accept CoveredCA plans, while the rest would not. The patients currently seeing the CoveredCA doctors will be permitted to switch to other doctors in the practice at any time, so their medical care wouldn't be "delayed" by the expected backlog of new patients trying to get in to see their new doctors. Patients currently seeing other doctors in the practice were told that they would need to change to one of the CoveredCA doctors if they moved from a private plan into a public one. They have dealt with Medi-Cal patients the same way for at least the last decade (interestingly, the Medi-Cal patients who currently see those doctors will NOT be allowed to switch to a new doctor, and will just have to deal with the larger backlog).