Hey I have been using a both UE4.72 and recently UE5.2 and heres a quick review from a product/arch viz viewpoint. (im also going to list the obivous pros and cons for those not in the gaming industry)
UE4 - almost perfect, stable, has just been amazing and replaced both vray and fstorm for me. (although i still use vray to bake round corners for unreal)
Con:
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Only artifact from caustics is specular bleed to near by objects when zoomed out, as far as i can tell there is no way to adjust this optimsation bias. But for 90% of situations this is a non-issue.
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DOF in reflections at shallow angles are incorrect but this is to be expected and can be solved with post production tricks.
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The need to optimise meshes… because displacement can only be used sparingly and POM has no self shadowing in raytracing.
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Lack of support for new plugins
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Primary rays Include DOF (for DOF through glass) causes a gridded artifact over specular pass. Not too noticable except in movie render so mostly have it off. (its mostly fine for hdscreenshot)
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Movie render queue cannot use DLSS, doesnt crash but breaks the image.
Other issues - has this version been discontinued? didnt work the last time I tried to pull from github. Which is a shame 
UE5 - Nanite and Lock actor plugin is tempting me to change. Infact general UI, usability, plugin support is a plus.
Cons:
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UE4 had more unified lighting, caustics, gi, reflections … etc e.g UE4 you dont need hybrid to see reflections of caustics where UE5 you need lumen or raytrace with hybrid reflectivity turned on both with their own pros and cons. Which isnt an huge problem but in UE4 you had one solution that fits all.
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Crashes… alot of crashes with right click… for seemingly irregular reasons. I have edits in the code for my build but I dont think thats the cause. Maybe I my build have issues… idk… either way its the most annoying thing about UE5
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PrimaryRaysIncludeDOF has same grid artifact as UE4
-Gotchas - BP_RTX (general engine control over-ride asset) has an annoying blueprint inside that turns off specular for lights every time it refreshes the viewport if nanite is enabled.
UE5 does not work with anisotropy - ie no simple brushed metal or carbon fiber. I guess something substrate will fix in the future?
Nanite meshes do not work with glass - but there are acceptable work arounds.
For archviz I would recommend UE4.72 for interior and product situations, for larger scenes you’ll probably need UE5. Dont forget engines and ddcs are huge and you may need 2 copies of each engine if you need plugins (in order to download the plugins via launcher thingy) On my machine UE4+UE5 is around 500GB (including ddc) (also not forgetting visualstudio and SDKs)-all of this needs to be on C drive is what I have been told so come to think of it, I havent tried this on another drive… either way be warned.
(other gotchas are the install times… its long… and be prepared to mess up and do it a few times)
Overall they both have pros and cons but compared to other engines, real time caustics is astonishing. Worst thing about unreal is the learning curve but its worth it for the speed.
I shall leave you with some high resolution screenshots from UE4.72 with dlss, all LOD0, og screenshot size 6k 30sec a img on RTX3090. Theres a variety of objects to show the technical aspects of NVRTX Caustics.
