What is controversial about thorium reactors?

It is being written as such, but where's the evidence of controversy?

Thorium is not particularly nasty stuff.  However, it is not to be treated lightly.  Does that disqualify it then as an energy source?

The question should be answered carefully, because there's a lot more thorium than fissionable uranium.  Secondly, there's enough thorium to last millennia.  You cannot say the same with conventional fuels.  Thirdly, its energy concentration is much greater than with solar or wind, meaning it will have a much smaller footprint.  Finally, the world needs an energy source.  Fusion is not available yet, and may never be.  There's got to be a solution somewhere, that is, if you want a non-carbon energy source.  Thorium as the solution has been waiting in limbo for over forty years.

What could go wrong?  The downside is the risk.  But if the risk is manageable, then why not?

There is a risk to many things in modern life.  Fire is a risk, but is acceptable because there is a long history of handling it.  You can die in a fire, but that is no reason to ban fires simply because you can get hurt.  Same is true of electricity.  You can balance off the risk with the abundant rewards.  It is the price for a higher standard of living for more people.

It should be noted that some confusion may be in evidence with the linked article, since the seventies experiment with molten-salt reactors did not involve thorium.  That is to say, you can run a molten-salt reactor without thorium.  They don't require thorium.  The ones they tested in the seventies used uranium.  The molten-salt experiments were a proof-of-concept only, and were quite successful.

Molten-salt reactors are definitely safer than solid-fueled water-cooled reactors.  Molten-salts have been used in solar thermal power plants.  It is an excellent way to store heat.  Why should it be controversial then to store energy from nuclear reactions?  The radioactivity has no effect upon the salts.  That is one reason why they are safer.  In comparison, using water as an energy storage medium for nuclear energy has its problems.  Water can be ionized by the radiation, thus producing hydrogen and oxygen.  Enough of that, and you can get an explosion.  This is what happened at Fukishima.  There is no risk of that happening with molten-salt reactors.  Indeed, the molten-salts tend to help moderate the nuclear reactions.  It is another reason for their superior safety characteristics.

Meltdowns are not a problem with molten-salt reactors.  Reason being is that they are already melted!  They are designed to run in that state, whereas solid-fueled reactors are not.  Once a solid-fueled reactor melts down, you have a problem.  Solid fuels are supposed to stay solid.  On the other hand, if something goes wrong with a molten-salt reactor, its molten-salts passively drain into a holding tank where it solidifies and is safe.

A tsunami like in Fukishima won't cause the problems that they had in Fukishima.  In fact, the reactor could be up and running again in a short time.  If there was any damage, it would not likely be in the reactor itself.

There should be no controversy, nor any doubt that molten-salt reactors are safer.  If they are safe for solar, then they can be safe for nuclear.