This really depends on your target audience but Zendesk allows for scalability that most startups need. You would need to gauge the level of knowledge needed for your agents - which is dependent on your startup industry.

For example, if you are a company like AirBNB, you may not need an extensive knowledgebase for your outsourced agents because the website users are not going to ask specific programming questions. However, if you are a technology company, your agents may need backend white-papers to help customers solve functionality problems themselves.