If it works as promised, this company has the potential to change the future of power. It's raised over $25 million, including investments from Marc Andreessen, Marissa Mayer, and Mark Cuban. Cuban, in fact, has called it a "zillion-dollar idea."

But futuristic startup uBeam, whose founder Meredith Perry says she is developing a transmitter that can charge phones, tablets, and other devices wirelessly, is now under attack. A former uBeam exec accuses the company of making false promises.

Paul Reynolds, former VP of engineering at the Santa Monica-based startup, has written a tell-all blog titled "Lies, Damn Lies, and Startup PR." In it, Reynolds likens the company to Theranos--a somewhat lazy comparison to another woman-led startup that has been formally accused of faking its product. Still, the point is clear: Reynolds, who writes that he has more than 20 years of experience working on ultrasound devices, believes the company has overstated its technology's capabilities and isn't close to delivering on its promise of having a working prototype later this year.

Lee Gomes, a writer for science and technology publication IEEE Spectrum, tweeted that Reynolds has confirmed he is the blog's author. Reynolds walked away from uBeam in October 2015, a little more than two years after joining. According to Business Insider, all of uBeam's original engineers have left the company.

The startup's goal is to create a charging station that uses sound waves to send power to devices within the same room. Perry, now 27, hatched the idea while a student at UPenn. She hired a team of 30 engineers and physicists, and they've been working on the product since 2011. The device hasn't yet been seen by the public. In April, a spokesperson for uBeam told Inc. that the company was preparing for public demos later in 2016.

Just last month, speaking at Imagination Day as part of the Tribeca Film Festival, Perry told an audience that uBeam has "proved out the technology, and we are on our way to deploying the product to the world."

In a statement to Inc. through a spokesperson Thursday morning, Perry said, "The company is heads down developing our wireless charging technology. We understand that with new products, there will always be a natural skepticism and ultimately we hope that the community will judge us by the product we release in the market."

This is far from the first time uBeam's technology has been called into question. When Perry announced publicly that her team had proved the idea back in 2014, one physicist wrote that the technology wasn't feasible, and that the money that has been poured into uBeam "illustrates everything wrong with tech investing today." Others have questioned whether the technology would cause health problems like infertility or cancer. In November, the mounting skepticism led Perry to give TechCrunch a closer look at how its technology works.

Cuban has publicly declared his trust in Perry and uBeam, even though he hasn't seen the finished product in action, telling Fortune, "I trust her enough that I haven't gone out and said, 'Show it to me.' She's shown it to enough people that I trust what's going on." Other investors who have seen the technology have been shown a minimally functioning version of it, according to Gomes.

It's one thing for a company to face skepticism from the public, but the newest claims against uBeam carry more weight, given their source. Perry and Reynolds both have made bold claims, and only one of them can be right. There's now more pressure than ever for Perry's startup to succeed--and to do it soon.

"They say that the most revolutionary ideas were considered crazy up until the point that they became revolutionary," Perry told the Imagination Day crowd in April. "If an idea is polarizing, it might just mean that you're on the right track."

The key word there is might. Soon enough, the company will need to prove it.