I've used FRA more times than any other airport in Europe. I can't say I've ever had a bad experience there. But I've mostly just flown into or out of Germany at FRA.
Only once did I have a transfer. I was coming from Munich before they had non-stop flights from MUC to Denver, so I had to change planes there for a non-stop flight to Denver. I came into an A gate and went out from a B gate and used the tunnel between the concourses. There was a lot of security along the route, because it was just 4 months after 9-11, but other than that, the change went smoothly. As I remember, I had an hour for the change and made it with time to spare.
I've only used the Sky Line once. That was in 2000, my first post-1900s trip, flying Northwest Airline, which used Terminal 2. I arrived at the airport in the Regionalbahnhof, which led me to Terminal 1, so I used the SkyLine to get to Terminal 2. The signs were a big help, and I made it without any problems.
My worst experience, if you can call it that, was in 2013, when we arrived in the extension wing of the A/Z concourse at, I'm pretty sure, gate A69, the farthest gate out on the concourse. The walk from there to immigration, at the base of the 'V', was half a mile. It seemed like it would never end. Then, although we had no checked luggage, we were led out onto the "arrival level", at baggage claim. I had never seen that level before; always before I came out into the departure hall and I was familiar with how to get to the Fernbahnhof from there. But I was not familiar with level 2, so I was sort of lost. Fortunately the signage was good and I found the way to the Regionalbahnhof, and then to the elevators to the bridge to the Fernbahnhof.
So, overall, No, I would not avoid FRA. I thought I was familiar with FRA in 2013, having been through there 10 times since 2000, but, due to changes in the interim, I did get a surprise. Since then, I have made an effort to become as familiar with FRA as possible, and would not be averse to using the airport again.