Should we have employee reviews at Startups ?

It depends on the size of the team. But I think they should be done even if over long period ( 6 Month ).

I think you don't need a very thorough review but maybe some sort of "trial" period or some time where you both try to understand if it's a fit ...

I would say that setting targets for employees (and making sure they understand them) and then later evaluating them is one of the clearer ways for both the employer and the employees to understand what the expectations are, and to be able to measure the results against those expectations. Else, how would you know if the employee is doing well, objectively speaking? Gut feel is very subjective:-)

Having a feedback loop with your employees is very very important, even if it is just positive feedback. Feedback does not have to be written. I have found that having a 30-min 1:1 every other week creates a good rhythm between the employee and their manager.

I have rarely been in a situation where there wasn't something that needed discussing.

Remember employees are people with feelings like everyone else. They also sometimes do not have the big picture understanding of things as the founders and thus need help thinking about things.

When neglected it is a problem waiting to happen.