I'm currently using 64 Audio Qis. I have used various Shure, Carvin, and Sennheiser universal IEMs as well as over the ear headphones and cheapo earbuds.
1) Resale - I can see resale being a possibility if I wind up not needing or liking them with universal fit.
IMO, if price is not a factor neither is resale. That said, you can still sell used customs, the manufacturer can make new molds for the new owner, so the difference in resale between customs and universals may not be that big anyway.
2) Flexibility - I can try different materials of tips until I find exactly what I like.
Maybe. Or you can try different materials and sizes of tips and still not find anything that works well. That was my experience.
3) Cleanliness- After a few shows, just chunk those flexible tips and put on new ones.
My customs have never needed more than a wipe with my shirt and a quick swipe with the "cleaning tool" for the sound holes, literally takes 10 seconds. Cleanliness is light years better and easier with customs.
4) Better seal - If the ear bud tip is expanding to fit your ear canal every time, it must be sealing up nice and isolating well.
The only thing I've used that gets a better seal than my customs is my shooting muffs. And they're not
that much quieter and significantly less comfortable.
I guess what I am trying to say is I need to be talked into molded buds before buying them.
You left out a big one. Comfort. I've never worn any IEM/earbud that didn't cause me significant discomfort after a while, some in as little as 20 minutes, others maybe an hour or two. All kinds of different tip size and material, didn't matter, nothing worked. My Qis can stay in for hours (8 is my longest run so far) with
no discomfort, pain, or fatigue. Honestly, given my experience with sticking things in my ears if someone had told me that I would be skeptical but they really have lived up to the comfort hype. Even if they didn't sound astoundingly better than cheapo IEMs the comfort factor alone would be worth it to protect my ears through a 4 hour show.