They are, indeed, very promising. However, have in mind that both those reactors are NOT being pursued for safety reasons. That is a side benefit (and ironically, a small one).

Molten salts are great at achieving high temperaturez, and cause less stress to the reactor in case you want to change the power output - a better match for the heavy renewable energy systems, and to use in high-heat industry. They have some ways to go in terms of proving technical feasibility in the long term, which will be a very long process withing the regulators.

Micro reactors are being targeted for some niche markets - remote.power generation (e.g. mining), low industrial heat, or other uses beyond energy (e.g. nuclear waste recycling/usage). Still a tough sell, as market is much smaller, and while they promise cheap prices through industrialization, you can also achieve it with SMRs.

Any talk you might hear about "we want to use X nuclear technology, because it is safer, and creates less nuclear waste" is usually a sales pitch aimed for public that have big misconceptions of it. And sometimes, in a very false and simplistic way (e.g. some micro reactors claim that passive cooling from Ambient Temperature is enough to cool down reactors. While this is true, it does not cover days where heatwaves can happen. Very likely, design will have to be changed to accommodate such High T days, as I highly doubt any regulator would just accept it as it is.

The true obstacle is the initial investment. Not even economics, which tend to be good- just investing in expertise, framework, and getting the financing costs at decent interest rates.

Nuclear is seen as very safe - including within governments. Most governments just don't want to take such investment, and decide to go on renewables, and crossing fingers for future solutions is just easier for them, as many tend to not plan ahead of their 4-year period. Much easier than trying to get controversial.

This is seen in today's news: UK, for instance, recently published that they are super invested in nuclear, and have a."very ambitious goal" of getting a lot of nuclear built by 2050. How much is a lot for them? Best case scenario: 25% of electricity capacity. Yes, only 25%.

Other technologies that are basically ready to be demonstrated also face market challenges. E.g. Thorium Reactor demonstration got halted, because they could not get a reliable source of fuel for their reactors, as there isn't a supply chain for them (yet), and not a big desire to create one. This is even despite the USA spending money to get rid of Thorium from other industries.

So while we are doing R&D for newer technologies, these tend to be for other uses that conventional nuclear is not suited for. As it stands, there is still a lot of development going on with conventional technologies, especially in Eastern Europe (SMRs with conventional designs) and many African countries (conventional Russian nuclear designs).