no one’s certain this will be cost-effective either

One of the great sins of nuclear energy programs implemented during the 50s, 60s, and 70s was that it was too cost effective. Very difficult to turn a profit on electricity when you're practically giving it away. Nuclear energy functions great as a kind-of loss-leader, a spur to your economy in the form of ultra-low-cost utilities that can incentivize high-energy consumption activities (like steel manufacturing and bulk shipping and commercial grade city-wide climate control). But its miserable as a profit center, because you can't easily regulate the rate of power generation to gouge the market during periods of relatively high demand. Nuclear has enormous up-front costs and a long payoff window. It can take over a decade to break even on operation, assuming you're operating at market rates.

By contrast, natural gas generators are perfect for profit-maximzing. Turning the electric generation on or off is not much more difficult than operating a gas stove. You can form a cartel with your friends, then wait for electric price-demand to peak, and command thousands of dollars a MWh to fill the sudden acute need for electricity. Natural gas plants can pay for themselves in a matter of months, under ideal conditions.

So I wouldn't say the problem is that we don't know their cost-efficiency. I'd say the problem is that we do know. And for consumer electricity, nuclear doesn't make investment sense. But for internally consumed electricity on the scale of industrial data centers, it is exactly what a profit-motivated power consumer wants.