Well, all open-world games have to load/unload assets. Unity's struggled with that part, though they've made some progress; it's possible to make a decent-performing large open world though it requires more effort compared to an engine where it was designed for that sort of thing from the beginning. (I think Rust is an example...I haven't played it myself, but it seems pretty highly-rated and complaints are about load times rather than in-game performance.) In any case it's difficult to imagine what this ADOM is doing to tank performance on larger levels unless they're literally just plonking down every single tile as a separate GameObject or something.
Yeah, I suppose my ire towards unity stems from games that have not been properly optimized due to such a low bar of entry into the engine, so we get a lot of great-high ambitious games that have shit performance, which really sucks for the end-user along with the dev that has to scramble to patch in performance as it were...
Whereas with unreal, I have had an extremely hard time finding an unreal engine game that has performed as horribly as an overly ambitious unity game, such as with eastshade for example, I have had one area boast a 90+ framerate, while just one step into a loaded section of a new area tank the frames down to literally 20 frames!
And I have a Ryzen 9 3900x|64Gb Ram|3090 and even at the lowest settings my frames still tank to 30 instead of the previous 20! I do hope that unity gets better, in spite of me despising that engine, too many indie devs do not, and a lot of great game ideas have been borne from devs that use that engine, so I hope it gets better...
...Or they switch over to Unreal! :)
EDIT: Oh, and ATOM is a small-scaled 3D isometric world so the performance should be exemplatory!