May I ask what obsticles you're running into?
First, I'll qualify that much of this just has to do with personal preference. Off the top of my head:
1) Web browsing - Safari is a much better mobile browsing experience, especially against Chrome. Syncing across devices is at the top of my list, followed up by ability to use a content blocker. Chrome does the former but not the latter. Firefox I just find to be an inferior experience. I like the Samsung browser--very, very similar to Safari but unless you are on a Samsung phone, you can't sync Chrome bookmarks (or at least I haven't discovered the setting that lets me do it). I spend lots of time in a web browser so this one is significantly felt.
2) Calendar. I use Google to sync my calendars across devices/platforms. There is one specific function I need to be able to do and bewilderingly, I can do it in the Apple ecosystem, using Google Calendar, but can't on Android, and that's the ability to move an appointment from one calendar to another. I have a set of different calendars on my Google account (Home, Work, School, Kids, etc) that my wife and I share to plan out our crazy family schedule. She often puts an item on the wrong calendar. On iOS, I just edit it and switch it to another calendar. Yet, in Google's very own calendar app, this can't be done. I have to delete the entry entirely and re-enter it. Really can't understand how Google lets this happen. Most third party calendar apps on Android are limited in the same way. I've found a couple that let me do it but their overall app experience is otherwise crap.
3) AnyList app. This is an iOS only app that we use for grocery and other shopping lists. It's great because my wife and I can share the lists and it offers a robust experience. What makes it indispensable to me is that it integrates with the iOS Reminders app. I can add items into a list on Reminders using Siri (most often on my watch), i.e. 'Add milk to my grocery list', and it automatically gets synced into the App. I'm likely going to continue to wear my Apple Watch even when I carry the Pixel so losing this functionality hurts. Worse still, I haven't found any way to replicate it on Android using Android Wear. You can do the same with Google Keep (at least you used to be able to, haven't checked) but only on the phone. Try it on Android Wear and the functionality wasn't there.
4) Messaging. The consistent 'elephant in the room' for many iOS to Android switchers. Of course I can just use SMS but it's an inferior solution and anyone who disagrees either isn't using iMessage to its fullest or doesn't have a large group of iOS contacts that I do to take advantage of it. Moving to something else will take a lot of work.
5) Other apps will less functionality, especially in regards to using FP for authentication. A bunch of apps still haven't been updated to use the FP sensor even though their iOS counterparts have. The Outlook Mail app's notifications don't allow me to delete Gmail messages as I can on iOS, just Archive. I also much prefer having all of my quick setting accessible from the bottom as is done with Control Center (well, at least it's that way on the 7, won't be on the X
). 1Password is a bit janky compared to iOS. There are scattered other examples.
I miss some aspects of 3D Touch as well...especially the cursor navigation and text selection. The implementation on GBoard just isn't as good.
There are plenty of things I like about the move though so don't want it to seem like its just bad apples. Notifications are cleaner and organized far better. Better integration with Google Assistant and Google Now is great. Google Assistant herself is much more useful than Siri. Selecting default apps is also missed on iOS.
Regardless, it's nice to have a new device to once again explore.