Learn Polish: Emigracja to bardzo trudna decyzja. - Emigration is a very hard decision.

We use trudna because it is an adjective directly describing a specific noun: decyzja (decision), which is feminine. The word trudno from this lesson is an impersonal predicate. You use trudno when you are stating a general condition and there is no specific noun being described (for example: Trudno to wytłumaczyć - It is hard to explain).

If I want to add "for me" to this sentence, can I use the Dative experiencer mi like in the rest of the lesson?

No, you cannot use the Dative case in this specific sentence structure. Because you are using an adjective (trudna) modifying a noun, you must use dla + Genitive: Emigracja to dla mnie bardzo trudna decyzja. You only use the Dative experiencer mi with impersonal words. To use mi, you would have to rephrase the sentence entirely, for example: Trudno mi zdecydować się na emigrację (It is hard for me to decide on emigration).

Why are both emigracja and decyzja in the basic Nominative case? Shouldn't "is" trigger the Instrumental case?

You are right that using the verb "to be" (jest) requires the Instrumental case for the noun that follows it: Emigracja jest bardzo trudną decyzją. However, this sentence uses the linking word to instead of a verb. When you connect two nouns with the word to, both nouns simply stay in their basic Nominative dictionary form.

Is it safe to assume emigracja is feminine because of its ending?