More than 5 years later my opinion hasn't really changed all that much.
If a migration happens, i would still vastly prefer a proper forumsoftware. I still like Xenforo, at least the one i am very active in. But i also see that Xenforo development has somewhat stalled. But in that regard it isn't much different that SMF 
My favorite aspect of XenForo is it's seamless scaling from portrait-mode smartphone screens to wide PC screens, and no reliance on a *completely* different mobile view.
My main gripe with SMF is the image upload and the quoting system.
Just pasting images from your clipboard to the forum is such an easy way of handling images.
The KiCAD Forum is one example of a Discourse forum that i know. And it's usability is atrocious. Just alone that it doesn't properly paginate and has an "endless" scrolling overview is crap.
EDIT: And if a micration to Discord, of all places, would happen, very likely me and a lot of other people will be out. Discord is a blight on the internet. It has killed so many open forums and essentially removed them, and all their accumulated information, from the internet. Discord is a closed system that you can only access with their client, and only after you have registered and are invited to the actual community. It's essentially a chat service on steroids.
Xenforo is a POS WRT remembering what threads you posted in.....get it wrong every time and only take you to the thread but never the latest post. 
SMF is in no way perfect but gets that right even for a thread that's 10 yrs old.
That is in part an inherent issue.
If you click on the notification for a thread, and new posts have caused another page to be created in the time since the notification was triggered, you haven't seen it all, unless you actually visit the following pages. Thus you won't get new notifications. I would not be surprised if there are workarounds for that though. And that does not affect notifications that are triggered if you are quoted or mentioned.
I am rarely participating in extremely active threads here, so i am not actually aware of how SMF handles that.
I mostly use the "Show new replies to your posts" link at the top, and XenForo has something very similar.
EDIT: This section doesn't apply to me here in this forum anyway, since i don't get notifications at all. So disregard this in respect to this Forum. The next section is still relevant though /EDIT
The time for how long the forum remembers your last seen post should be configurable by an admin. I had that discussion in a XenForo driven forum. The admin did not want to increase the time too much, because he was expecting database bloat if the last visited postition is never purged.