http://apathy.i2p/articles/wallet-of-satoshi-spark-beta-review
Transactions are still processed by a federation, the federation can always refuse to process any transaction, lightning payments sometimes fail to route through the network, and there is still a small risk of theft. The Spark protocol was criticized by Bitcoin Magazine and Bitcoin Layers for stuffing a trusted federation under the mattress. If you receive funds over Spark, and the previous owner colludes with the federation, your funds can be stolen.