You can do higher without dual unity, at least on SC. On HC it's a whole other issue, never gonna get super high on certain classes by design and yeah you're gonna need very specific gear. I mean dual unity is the hard part but you also need the legendary follower item to make your follower immortal, last season I got 2 unities before I saw a follower item(though was very lucky on unity drops obviously), so really you need all 3. And probably some anti rip necklace too. And then you need to farm a good RoRG and stuff, it's quite a grind to get actually good gear for high grift progression and there's no shortcut from trading(though playing in a 4class group will help a lot with that).
On the other hand if you just want to casually fuck around, D3 is a lot more fun than PoE I'd say, it's fast paced earlier, a lot of the skills are more flashy and more powerful and combat flows very well. Downside is how limited you are in build selection until you get specific items, while in general PoE offers more build diversity based on gem choices only, with uniques enabling more builds or reinforcing builds, rather than being the builds like legendary/set items are. Playing before you have 5pc of your main set(minus classes that have 4pc sets like monk) is pretty boring since there's not many options that are efficient, and then once you have the set, you can really only play the one build that set enables. This patch they added more sets so that's great since it means you can do more than one thing per class(well for the most part, but some classes already had more than one playstyle) but it's still pretty limiting.
Anyway very different games, people like or dislike them for different reasons and it's pretty much opinion I'd say, I think both games are good and fun and both games have major downsides to them too, and you juggle that as you please. Great thing is you're not forced to play one or the other all the time, you can alternate and shit. Seasons/Leagues are great for that.
Don't think I'll play this season though, we'll see, but I have some backlog of single player games and PoE beta/DFO beta are probably going to take all the rest of my time. And then this summer, FFXIV, so yeah not sure when I'll have time for D3 again. Hopefully they have another expansion coming, this will get me to play.