Call me a heretic, but even though I love my retro gear, I can't think of a single thing that can't be done at least as well by modern hardware, pragmatically speaking.
I mean, I see it a bit like why some people keep enjoying vinyl records. Given the fact that either high quality analogue R2R tape or digital recordings will be closer quantifiably closer to a studio master (analogue or digital) than a vinyl pressing can ever hope to be, why do people want to deal with vinyl's limited dynamic, noise, susceptibility to dust, etc ? Likely because of nostalgia, appreciation for the specific sonic signature that results from the constraints of having to master for the format, the inherent tweakability of the process, etc . (EDIT: Full disclosure, I am not really much into vinyl currently, so maybe I'm missing the point in part or in whole).
I can fire up DOSBox on practically any decent modern machine with an X2GS connected and with Munt installed and get a more fluid experience in Doom, Space Quest III, etc and get better fidelity sound and music than on "period correct" or at least baremetal compatible hardware, but that's not (usually) the experience I am looking for. I could also play an "enhanced" source port with higher resolution, etc or sometimes a modern remake (if available), but I don't (usually) want that either.
Sometimes, I will appreciate having access to Munt running on a Pi, an X2GS, emulators, overkill hardware for running old software, an Orpheus sound card and whatever modernish niceties my fickle mind desires (and my wallet can afford).
To try answer the original question, the only thing that is not "better" in modern hardware is the lack of variety and oddball experiments in mainstream PC computing hardware. Everything is getting commoditized and mass market. Sound cards are mostly DACs and differ only in output specs, video card chipsets come from 3 companies, CPU upgrades accross more than one or 2 gens are a thing of the past (as are weird and wonderful things from the likes of PowerLeap, etc).
So, in other words, everything is now "better", but a lot of the fun and quirkiness of old is gone and likely won't be back, ever, at least on the hardware front.