The Alarm Expression reliability is poor when compared to generating the event in the controller. Since OPC DA feed is polled on 1000 ms interval the accuracy of the timestamp will be 1 second or worse.
The softalarm service is capable of handling expressions in the range from hundreds to a few thousands, but its hard to tell for sure since the OPC DA server behavior is involved.
If the OPC DA server is "slow" to add items, a large configuration (1000+) may result in an overall unreliable alarm expression functionality.
I sincerely recommend generating the events in the controller rather than the HMI.