??? searching for variable instance and log is independent from using hibernate search or not. Unless you want full text search in the variable values. In that case I think you have to much of your domain model IN jBPM.
Pesonally I store only very limited amounts of my domain model in jBPM. Only info needed to steer the process.
If I want to search the task instance and process instance by the business variables, but I worry about the performance of searching the VariableInstances table by the business variables. You advise I have to store very limited amounts of my domain model, but the process instance become more and more, then the VariableInstance table's size will be bigger.