Check how many hospitals and doctors are available with these plans. My understanding is that this is a problem with marketplace plans, exacerbated by ACA mandates and in rural areas.
It's a good point - before buying an insurance plan, make sure the provider network includes the doctors you want to see.
The reason it's happened is not what I would call a "problem" or directly attributable to ACA mandates, not exactly. It's because insurance companies have been trying various ways to reduce the cost of care, and one way they do this is by squeezing the providers to agree to lower fees. As the provider networks get squeezed, the larger, more successful, and more powerful ones decide not to sign contracts, and they get excluded from the provider network. So a lower-cost plan may be lower-cost precisely because it has a narrow provider network.
Two ACA mandates have contributed to this problem, sort of. The ACA implemented "essential health benefits" which are required for all plans to be sold on the marketplace. It includes 10 things like hospitalization, ambulatory care, maternity care, etc. Details here:
https://www.healthcare.gov/glossary/essential-health-benefits/ Some states, through their insurance departments, implemented EHB's for all plans sold in the state. This raised the cost of care, because the benefit plans were larger.
The other contributor to the issue was Medical Loss Ratios. Another ACA rule, this one said "no more squeezing your customers; insurance companies have to actually spend the majority of what they collect in premiums on actual care." So, in a post-ACA world, an insurance company can't cut your benefits, and they have to actually use 85% of the money you give them to provide benefits to you. I know, how awful! Stupid Obamacare! The net effect was to squeeze providers because they couldn't squeeze their customers as easily.
TL;DR: Insurance companies lost some of the levers they used to have available to drive P&L, so now provider networks are more narrow than they used to be.