As I mentioned in another thread, it's impossible to say which is more transparent, online or brick-and-mortar, unless a more detailed investigation is conducted, a thorough audit conducted by someone qualified for this purpose.
Slot machines in physical casinos can be modified - the transparency is not so clear, I think manipulation can happen in any physical casino. However, due to advances in technology and stricter regulations and oversight, modifying such slot machines is now nearly impossible. All machines are tested before launch - a mandatory step for any casino to maintain its reputation.
Given the increasingly stringent government regulations and oversight of the gambling industry, every business owner must comply with the rules. If they are found to be in violation during an audit, they could face severe penalties, including license revocation and substantial fines.