It's the super steep >20% bits within a climb which are bloody murder, especially on road bike gearing (even modern 11-30 or 11-32 cassettes).
This one is nearby and it's about what I'd voluntarily do again to myself on road bike gearing; it's not the 10.3% average for 2.17km what is the problem but two stretches of >25% where it feels too steep/unstable to get out of the saddle and grinding it in the saddle is just horrible.
https://www.strava.com/segments/16532332
Conversely, this one is pretty pleasant, despite being 3.3km long at 9.5% average - the steepest parts are maybe 15% which is far less daunting - just get out of the saddle and go:
https://www.strava.com/segments/10824496
However, mostly my favourite training climbs are a constant 6%-ish. Oh, and if you're ever in Croatia, I highly recommend doing this one:
https://www.strava.com/segments/9475334
It’s a decent climb (1750 elevation gain), but it's mostly a constant 6-ish% with a slightly steeper bit only at the very end and a flatter bit around the middle. The view is stellar when it's a clear day.
I currently run a 50-34 with a 11-30 which I find to be a happy compromise on 11 speed for the local hills, when I convert to 12 speed it's probably going to be a semicompact and a 11-32.