A messaging app that has drawn comparisons to Facebook’s WhatsApp has already raised $850 million in advance of its initial coin offering, and is seeking a second round of funding before launching to the public. That would make it the biggest ICO in history.

But reports are surfacing questioning Telegram’s plans, with some financiers questioning its technology claims, the lack of a revenue plan for the service, and concerns on who is using it. The venerable Forbes called it “the scam among cryptocurrency scams” in an article this week.

Telegram earlier this week confirmed with the US Securities and Exchange Commission that it has privately raised $850 million. It is also quietly seeking a second round of private financing before launching its public ICO fundraising on a date to be determined.

Last year saw ICOs raise more than $4 billion in funding. An initial coin offering provides tokens for use in various company ecosystems, with some listed on exchanges and increasing in value. Unlike traditional intitial public offerings, no company equity is surrendered in an ICO fundraising, making it a favorite of startups.

The Telegram messaging app is highly regarded by the cryptocurrency and blockchain elite for its privacy features and security, resulting in its huge pre-ICO fundraising.  It will reportedly use the funds it raises to build the TON Network (for Telegram Open Network), a rival to the ethereum platform, the blockchain that currently underlies many of new online companies.

Telegram ICO investors will not receive their TON tokens until December 2018 at the earliest, which means it will not be listed on an exchange until 2019. The funds lock-up has caused skepticism, and the fact that the technological claims of its white paper are unproven has also raised eyebrows.

Adding to concerns is that the Russian-owned app (founded by Pavel and Nikolai Durov) has been a target of the Kremlin, which wants to be able to hack its encryption, also troubles some. Some have accused Telegram of being a messaging tool favored by terrorists, child pornographers, and other unsavory characters.

Still, a valuation approaching $2 billion is likely even before its public ICO. That will buy the company a lot of time to address concerns.