What's the hardest part about writing a good email?

None of those. The hardest part is accepting the fact that your first draft is going to need revisions (probably many of them) once you start using that email. I've got many years of experience writing email sequences as a copywriter, and although I can prepare a first draft based on reasonable and tested assumptions, I always keep in mind that I'm going to have to make changes based on testing if I really want the best version possible of that email. You see loads of discussions about "What's the best subject line?" or "What's the best intro?" The truth is, nobody can answer that question — only testing can give you the answer. What's more, sometimes you start off with a proven assumption only to find out that in this particular scenario it flops. And then you have to investigate exactly why it didn't work, and learn from it.

Everything @cedricdebono has told is so true! In fact, with all the "proven templates" or tactics out there, you can't simply take one of them and hope it will drive the same results for your business. There are so many aspects that could influence your email success - from the audience specifics to the campaign timing. Also, best practices might not always work equally well - sometimes you need to step away from the ordinary to stand out from the crowd and catch the prospect's attention. So, trial and error (and lots of A/B tests!) is the only way to find what works best for you.

hello

it is difficult to write in the correct tone, because the reader's reading or interpretation may not be correct

When it comes to cold outbound, I personally find it hard to be concise. Lots of folks mention that shorter messages get better response rates. When I receive a short message, it feels like there was no care behind the text. It feels impersonal, curt, and low-effort. This feeling extends beyond email for me. I even text in full paragraphs with grammatically sound sentences! I know I'm in the minority. I'm still working to mentally get past it and understand that most people don't read into it as much. ¯\_(ツ)_/¯