This is an excellent political move on behalf of the administration. They can take this step to appear in favor of clean, safe, nuclear power, with very little risk of being responsible for the creation of a new nuclear power plant. It removes none of the real obstacles to actually building and bringing a plant on-line. It almost doesn't matter what the funding sources are or how secure they are, attempts to build a plant will still take place in an environment that is very highly regulated, with regulations that frequently shift, and numerous avenues of legal delay available to people who wish to block the effort.
Certainly, encouraging the building of new, practical, energy production is a good thing. However, the reasons many previous plans have defaulted, and the plants they were to fund never brought on-line, are not resolved. Addressing the obstacles created by our legal and regulatory environment would have done far more to actually create a new, producing, plant.
In other words, there are reasons people aren't willing to risk their own money to build these plants. While loan guarantees do encourage people to loan money, it's easy to convince people to risk someone else's money and does nothing to correct the reasons they weren't willing to risk their own funds.