What is "reasonable range" for value of nis.unlockedLimit

Hello,
config.properties file says:

Maximum number of unlocked accounts. Meaning: maximum number of accounts that are allowed to use this NIS for harvesting
Keep the value within a reasonable range, a too large value an cause problems for all harvesting accounts.

I am running supernode and would like to know what is the highest safe value.

The default value is 4 which is kind of reasonable. But a value of 8 is ok too. Your logs will be larger though since every attempt to harvest with an acoount is logged.

Why is there problem in the first place?

There is no real problem, the logs just get too big and you should give a user an incentive to run his/her own node.