Fedimint is kind of like Liquid without the blockchain. Liquid custodies the pegged-in bitcoin using a miltisig quorum of functionaries. The functionaries also take turns building blocks and validating the blocks that other functionaries build. The blockchain is public so anyone in the world could be surveilling your liquid transactions. You can use confidential transactions to conceal the amount and assets of your transactions but the transaction graph is always publicly visible. I don't understand this feature at all because the transaction graph is more important to your privacy than the amounts. If you have some Liquid tokens and the network goes down or turns malicious you get rugged.

Fedimint has a multisig quorum of guardians. They control the bitcoin deposited at the mint and collaborate to validate each ecash transaction. But there is no blockchain, which means there is no public historical record of transactions. There is also currently no way for an outsider to audit the supply of issued ecash (the guardians audit it with every transaction). This sounds bad but there are upsides to the trade-off. Ecash has far superior privacy properties compared to Liquid. When you send ecash to another user the Fedimint guardians (and everyone else in the world, for that matter) are totally blind to the identities of the parties that are transacting. They only know the amounts involved. If you have some ecash and the fedimint goes down or turns malicious you get rugged.

There are a lot of other differences but hopefully this answers your question.