The question asks about hidden costs of OER's, but the real hidden costs are the ones associated with non-free texts. Students are forced to pay exploitative amounts of money for a text, but this is hidden from the instructor, who typically has no idea if the book has gone up in price since last year.
STEM students are also forced to pay large amounts of money (typically $100/semester) for online homework systems like MasteringPhysics or MyMathLab. These were originally quite cheap, but then the publishers jacked up the prices year after year. Again, the endlessly rising cost is hidden from instructors. There are open-source solutions such as LON-CAPA and WeBWorK, which can easily be provided to students for free. However, book reps steer professors into hitching their wagons to their own outrageously expensive systems. This also prevents students from buying used books, since only the new book comes shrinkwrapped with access to the software.
Because many OER's are created without any funding structure, they don't receive periodic upkeep/revisions as commercial texts do. Hence, they may be unreliable as a resource.
There are some reasons why the opposite may be true. The market incentivizes authors and publishers of non-free books to make trivial or cosmetic changes, but not to make substantive changes. Cosmetic changes are good in the marketplace for the same reason that they're good when you're trying to sell cars. Frequent trivial changes, such as renumbering homework problems, are also a big win because they prevent students from using a used copy of an older edition.
I also don't think commercial textbooks have a good track record for staying up to date. I'm a physicist. An example in my field is that professional relativists decided many decades ago to follow a convention in which mass is considered to be invariant, i.e., it doesn't depend on one's frame of reference. Authors and publishers of textbooks, especially at the college freshman level and lower, are only now beginning to adjust to this convention. This is because there is no commercial incentive for them to change. In fact, there was probably a disincentive, because they don't want to annoy professors who have gotten used to teaching things according to conventions that date back to the Roosevelt administration.
OER's undercut commercial products and therefore hurt job opportunities for people in publishing industry (authors, editors, etc.). Hence, there is a financial cost to adopting OER's, paid by those who lose job prospects.
Yes, and that's a good thing. When the automobile came along, jobs were lost by workers who made buggy whips. It was a good thing that those people lost their jobs. Capitalism isn't a permanent license to keep on doing the same thing forever in the same way. This is the concept of creative destruction. In a system where everyone is forced to pay subsidies to buggy whip manufacturers, society gets poorer over all.
Pressure to use OER's (from colleges and local governments) creates tension inside academic departments because it restricts academic freedom. Therefore, instructors may be told to use a textbook because it's free, and not necessarily because it's good.
Institutional and government pressure to use OER's are very weak (although they do exist in some places -- see note below). Institutional and government pressure not to use OER's are very strong. And it's fallacious to argue that openness is somehow opposed to quality. They're orthogonal.
What really restricts academic freedom is when a department insists that all faculty use the same text. Publishers are very good at getting faculty locked in to a specific text. For example, they get them hooked on a specific online homework system, or other ancillaries such as the slides that they provide to go with the book.
Using texts that cost money also sets up a dynamic that is opposed to academic freedom, because students buy a certain text, which costs money, and there is then a feeling that you can't change texts, because students have already bought the text for the first-semester course, and now you can't ask them to buy something different for the second semester of the sequence.
OER's stigmatize authors in academia from creating materials and taking them to market. Hence, authors may no longer feel that selling their own texts/materials is appropriate, receiving shame for not offering to give away their work.
Citation needed. I haven't seen any evidence of such a stigma.
Note on institutional and government pressure to use OER's
There was a series of comments debating whether any such pressures really existed, with many people expressing skepticism about their existence. They do exist in some places, but they're rather weak. I teach at a community college in California. Democrats in the state legislature -- bless their little socialist hearts -- have tried hard to legislate the creation of OER's and encourage their adoption. State senator Darrell Steinberg has pushed this hard, as did (moderate Republican) Governor Schwarzenegger and Governor Brown. Some relevant legislation was SB 1052 and 1053. There was a lot of confusion and milling about because the politicians were not particularly connected to the reality of the classroom, and commercial interests saw this as a way to promote their own interests. E.g., Apple saw it as a way to convince K-12 schools to buy an iPad for every kid. At the post-secondary level, the net effect so far seems to have been that there is some very gentle encouragement at my school to use OER's. Workshops have been offered. Every semester, my department asks me in the textbook adoption form to state whether I'm using a free textbook in my course. When I check the box saying yes, they respond by putting a little icon next to my course in the schedule of classes (which students never actually read, because they search for classes using a web interface). The main thing that IMO has been beneficial about all this is that it's at least made clear that it's OK for instructors to use OER's -- which had been very much in doubt before.