Spreadsheets are computer programs where the typical user doesn't usually care about the problems you mention as long as he gets the correct results.
The pros and cons I had from my reactive programming experience are:
Pro 1) Terse code: My typical defect/kloc remained about the same. So by using about 10x less code to do the same job, my defect count went down proportionally.
Pro 2 ) Live development: No need to stop/compile/start to change a rule, so it's possible to view the result of a rule change quickly. (this also leads to Con 1: Capacity to screw up)
Pro 3 ) No need to repeat formulas: Applying a formula to all the rows in a spreadsheet is tedious.
Pro 4 ) Complex Relationships: It's quite challenging to implement complex object relationships using spreadsheets as each object requires its own worksheet.
Pro 5 ) Data Visualization Tools: Reactive rules encourages developers to clearly layout their data and a good tooling suite can allow developers to visualize everything.
Con 1 ) Capacity to screw up: With live rule editing, a mistake in a rule could get evaluated instantly and potentially corrupt the application data beyond repair. This can be partially solved by good development practices such as taking backups during development.
Con 2 ) Fan Out: As the parent mentions, how many rules are triggered on a data change? This can be solved this using static analysis of the rules to figure out a triggering graph.
Con 3 ) Latency/Synchronicity: A good infrastructure can enable developers to define rules that synchronous, asynchronous or delayed execution. It's also possible to have an infrastructure that allows developers to monitor the cascade of execution. However, each change is executed through event triggers. A large number of changes leads to a large number of events which adds overhead. It wouldn't be a good idea to program a video game with this for instance.
Con 4 ) Large Data Set: Changing a reactive rule on a large data set might lead to a chain of event that takes a large amount of time to execute compared to a customized SQL script that would perform the change in a single batch.