So many problems. First off, you can only register DMNSeCard on one device at a time, so if you have a family membership, and, say, one parent who doesn' t have DMNSeCard on their phone wants to take the kids — you'll have to deregister one device to have it on the other person's.
There is no true login screen, so to "login" you are instead sent to a registration screen, which is confusing because it makes it seem as if you are creating a new account, even if you already have one. You even need to type in the password twice AND know your member number — just to log in! And if while logging in you forget your password, you'll never get the reset email that allows you to change the password — you'll need to go to the site from a computer or browser.
DMNSeCard also requires you to be online for it to work, so say your cell reception drops, which can happen frequently while IN the museum — you may have problems accessing tickets or getting into exhibits where a card is required. As a result, make sure you open DMNSeCard BEFORE going into the museum and keep it open just in case your reception drops and DMNSeCard no longer becomes accessible.