I've been working on Mainframes for the passed 8 years full time, and several internships before that. I often go several days with out needing to use ISPF. I use
https://compuware.com/workbench-mainframe-modernization/
for dataset editing, job submission, etc. I find the Unix shell you access through ISPF to be very difficult to use. Luckily SSH is available on z/OS, so if I need to do Unix stuff I can just SSH in with the client of my choice.
The biggest difficulty I had when getting started was understanding JCL. I had come from a Linux background, so was used to executing commands with arbitrary file names, looping, etc. It took me a while to understand how JCL worked. Once I understood that you have to define datasets with things such as space allocation and disposition before using them, and that JCL was a series of sequential steps with the output from a prior step available to a later step a switch flipped and I went from struggling with JCL to fairly comfortable in a couple days. I'm not sure how much of this was the fact that I was an intern at the time so didn't have a lot of programming experience and how much of it is the platform. I find that once you get comfortable with the basics of JCL the structure it forces you into actually makes it easier, even though it is limiting in some ways.
The issue I still have with the Mainframe is documentation. Unlike most programming questions a google search does not turn up Stackoverflow results. I find I spend a lot of time going through random forums or reading the IBM doc. I don't find IBM doc easy to understand, but the more I have to deal with it the easier it gets.