Slack’s recent $200 million fundraising, at a 36% higher valuation than a year ago, is a reminder that investors remain big believers in the fast-growing chat app. Slack not only aims to win the niche of chat. It also wants to be the main way that people communicate with each other and with the various software systems that make up a business.

But Slack may find it hard to justify its $3.8 billion post-money valuation if it doesn’t fix some longstanding issues with its product, its technology architecture and its back-end features. These issues have made it impossible for companies with tens or hundreds of thousands of users to officially adopt Slack—even when many employees are asking for it. Uber, Twitter and LinkedIn have each tried using Slack and pulled back, say people close to those companies.