Reddit reviews Hostages of Each Other: The Transformation of Nuclear Safety since Three Mile Island

The big take out was: we need to talk to each other.

Sadly we already knew that after TMI: an accident on your plant can impact my business, so we are all "Hostage of each other. TMI lead to the creation of the Institute of Nuclear Power Operations.

Chernobyl lead to the creation of the World Association of Nuclear Operators.

It's far from just a training problem. The design was faulty, but other RBMK had corrected some of those flaws without sharing it. There was also a big organisation problem: the safety was subordinated to the operation; after Chernobyl the safety was directly reporting to the head of the reactor.
The secracy was also rampant. Of course the USSR kept a lot of information even from different people internally, but for instance there was very little documents in the control room, operator had their training but could not keep the material for fear of leaks. Today everything is obviously available from the control room to assist decision.

From the consequences point of view, the United Nations Scientific Committee on the Effects of Atomic Radiation existed decades before Chernobyl and the accident provided valuable scientific data to them.

Here's what the WHO has to say about Chernobyl: "The Expert Group concluded that there may be up to 4 000 additional cancer deaths among the three highest exposed groups over their lifetime (240 000 liquidators; 116 000 evacuees and the 270 000 residents of the SCZs). Since more than 120 000 people in these three groups may eventually die of cancer, the additional cancer deaths from radiation exposure correspond to 3-4% above the normal incidence of cancers from all causes."

Here is what they say about Fukushima: "The present results suggest that the increases in the incidence of human disease attributable to the additional radiation exposure from the Fukushima Daiichi NPP accident are likely to remain below detectable levels" or "In contrast with the findings of only marginal internal radiation contamination among children and adults, it appears that the increasing burden of noncommunicable diseases and mental health problems may outweigh the burden of disease caused directly by radiation.

I let you judge if that count as learning from Chernobyl.