Well-implemented cubemaps have been the bread and butter of excellent environment lighting for decades, and it’s remarkably flexible to boot. To your point, getting actual transparency out of UE is almost unaffordable in very many cases: the forward shading direct lighting alone is pricy, nevermind the sorting and depth peeling to allow for RT effects.
You’re making me think: is there any way you could fake a transparent effect in UE by having an opaque object that derrives reflection from lumen traces, but the ‘refraction’ is just the lumen probe lighting with some normal perturbing and blurring? It would draw as opaque, get accurate front-facing reflections, and still have a possibly usable simulation of refraction. I have no idea how you’d go about implementing that however.