I'm the opposite. I've only been riding on Zwift regularly since November, but so far I'm loving the big mountain climbs. It's one of my favorite aspects of the game, actually. I live in a flat area so it's my only experience with extended climbs, outside of a couple of MTB trips to Colorado a few years back. The only time I ride the flatter routes is if I'm participating in a group ride or doing workouts (in ERG mode). I've only done the Alpe once but am already looking forward to doing it again, and will probably try Ven-Top soon. I love the Radio Tower climb, and especially the steep bonus climb.
The Alpe is a tough ride mostly because of how long it is. It's not as steep as other climbs, but it never really lets up. The day I did it, my total ride was 1:45 (1:18 on the actual climb, the rest being the ride in and the descent back down). I spent nearly an hour in Zone 4 or higher on my power meter. You can dial back the intensity by lowering your trainer difficulty, or just slowing down on the climb. I also see people running workout mode up the Alpe which (assuming ERG mode) which makes the terrain irrelevant in terms of effort. I'm not sure I see the point of that, but hey... whatever spins your wheels I guess.
Any tips from others on climbing Ven-Top? How does it compare to the Alpe? I've read that it's tougher and I'm assuming longer as well, so I'm saving that for a day when I have lots of time for riding + a nap.