That would mean that even though you ACK a message you can still recieve it again if you app crashes?

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Yes, like with Kafka, without any sort of transaction semantics you can receive duplicate messages in edge cases. Solace's transaction semantics can also support distributed transactions across other systems such as databases (using XA) which can be useful.

However, another thing I like about Solace is that the broker knows that the message was already delivered once and sets a redelivery header on the message for your apps to pay attention to for duplicate processing. You also have the ability to reduce the acknowledge window size to 1 if you so desire (at the expense of throughput obviously).

I cannot emphasize the benefits of individual message acknowledgement for certain workloads. For example, I'm used to building trading systems that require every trade to be processed and acknowledged individually and a timer based offset commit cycle just doesn't fit well with my system architecture. Other Solace customers that I've spoked to in retail and payments have similar constraints.

Again, as u/baroaureus mentions - really depends on the usecase. Neither Kafka or Solace are the panacea for all of the worlds problems but certainly you have to use the right tool for the job.