Over 40 years I've worked in many companies, large and small and can say without reservation this is a well run company doing highly regarded work in a challenging field.
Cons
Startup with the caveats that go with early stage companies.
friendly environment, give you lots of responsibility, high level of transparancy, good opportunity for getting publications
Cons
internship is unpaid and very demanding - for how much they work you it is unbelievable there is not even a minimum wage approved - opportunities for advancement also seem limited as there is not much of a junior staff