Most UBDs are HDR, but the HDR isn’t done well (especially for older releases).

Also, since most UBDs are older releases, it would be upscaled from 2K or less. Even ones shot on film would’ve most likely had a 2K (for BD) or even reusing 1.2K DI (for DVD, yes, that happens, e.g, bourne M1 looks as sharp as 1.2K on UBD).

Do they require different software or a much more powerful editing rig?

Maybe, but you needn’t a more powerful PC to edit. Many editors commonly edit with LQ clip proxies which they then should substitute for HQ clips before the final render, except when they forget.

Eg. EX_MACHINA: text on BD looked sharp, but the footage looked DVD-quality. Probably a case of forgetting to un-proxy the footage. UBD looks 2-3K quality though.