For me, it depends entirely on the final design. Does it look like it work? How does it inform someone of the character that wields it or: what's the story of it? What's the context?
OT sabers are wonderful because they are based on things real-world things that were built for a specific function; flash handles were meant to work. From that, it already has what I call a "build logic", where the parts are where they are because they need to be there, and that can be used for inform alterations made to them.
The PT kind of lacks that but I don't necessarily poo-poo them out right because of that. They lack the robust qualities of the OT sabers but, having said that, my two favorite lightsaber designs from the PT are Dooku and Palpatine's (particularly his V2, the one he fights Yoda with).
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The more artsy, organic/ancient sabers is where I have more qualms with. More often then not, they go overboard in designing them; preferring form over "function" (Kylo Ren's a good recent example). They tend to have too many things on them, jetting out from the sides or something littering the emitter, where I think the idea of keeping it simpler is the better. Again, they're not all like that, some fan interpretations of these sabers (some of them posted here) are really good and they fit in this categtory; I think you just have to know when to stop.