Is your wallet large? Does it have a lot of addresses and transactions? Have you done anything atypical with your wallet such as importing scripts and pubkeys?
The migration process shouldn't take too long, although it can take some time depending on the size of your wallet since it has to rewrite everything to a new file.
It's possible that there is a bug somewhere, although it may be hard to diagnose that without access to the wallet itself.
Wallet is about 2^6 transactions and about the same number of receiving addresses. I haven't tried to import scripts or pubkeys, but in the last days I tried to install the Berkeley DB dependencies to run bitcoind from the terminal (on a Ubuntu PC). bitcoind still couldn't load my wallet. Everything worked fine with bitcoin-qt UI.
NEW: I reloaded my backup wallet. I saw my transactions and balance again but I did not send nor receive. I shut down bitcoin-qt. When I started it again, it started with some bitcoin-25.99* version, and no wallet was loadable. It was only a node with peers, nothing else. I resinstalled bitcoin-25.0 and finally I could access my wallet again. I successfully made a new transaction with it. It says "*.legacy.bak". Migration apparently didn't work.
At that point I'm just too afraid to try migratewallet again. I don't want to end up with a corrupted wallet file. I'd really want to use bitcoind though but it doesn't seem possible with a legacy wallet.