Really forward-thinking for its time. Ferenczi's theorization tends to travel an ambiguious space with a lot of ease and in a very modern way. Let's list a few ideas to paint you a picture:
1. A discussion on the important link between omnipotence, psychosis, and the mother decades before Winnicott.
2. The recognition of symptomatic approximations towards sexuality IN GENERAL, speaking of homosexuality and heterosexuality alike, as well as an argument in favor of more integrated (bisexual) sexual identities (which is pretty amazing for a classical psychoanalyst).
3. The introjection of the outside world, in virtue of the acoustic/corporeal relation to it, as means of representing through language and the body.
4. Language as ACTION (!!!) which is formally conditioned by unconscious complexes. This is especially post-modern and has far-reaching consequences.
Of course, much of the collection is stereotypically kooky, in line with classical psychoanalysis. There's many examples of supposed phylogenesis and the universal equation between paranoia and homosexuality, to name a couple. These are mostly artifacts of their time, and I think we can cut Ferenczi some slack in this regard. He was a brilliant thinker.
As classic as Freud writings: it evens feel like appendices from him.
Of course it ain't perfect, it has the same exact vices as the early psychoanalysts, but Ferenczi sometimes goes beyond and criticizes the capitalist system (without marxist basis) in pure libidinal and mental terms (see the last paper of the present collection 'The Ontogenesis of the Interest in Money').