I had the idea for a UW list when I realized my only black spells were Glimpses, Crypt Incursion, Fatal Push and Collective Brutality, with Damnation and LLotVoid in the side, and white generally offered better stuff (Verdict and Settle, RiP, Path), and sideboard options. It definitely still needs tuning, I've probably only played about 12-15 matches with it. Also, I was mistaken - currently running no creatures. I'm still somewhat learning how to play control (I'm familiar with it since I've played MTG for 15 years) so advice on the control package would be good.
The basic plan is to slow the game down with counterspells (Cryptic is so boss) and removal (Path, Verdict), and get some combination of Trap/Briefing/Orb/Fraying Sanity down and protect. It seems to work well, your opponent seems to settle into a grindier game and then you hit them with a Trap > Briefing for most of the rest of their deck. Also, spicy haymaker in Psychic Spiral, haha, but it's an instant which is key!
Firstly, lands seem to be good colour wise, but with the prevalence of Field of Ruin around, I haven't had a Shelldock survive to activation in some time, so I'm considering cutting, or dropping to fewer than 3. Academy Ruins is great for recurring Orbs, but colourless mana (on top of the stuff from our own FoR) can sometimes be hard. I've just thought about an Ipnu Rivulet as an option too, but with no Crypt Incursion, lifegain is a little harder to come by so I'm cautious. I'm also thinking about going up to 23 or even 24, we really want to be making our first 3 (ideally 4) land drops.
The only loss to milling from taking out black is Glimpse, and with Mission Briefing doing a decent job acting as a mill spell with Archive Trap in the yard, as a 4-of with 4 Field of Ruin, it fills Glimpses slot pretty nicely.
For counterspells, I think 2 Logic Knot and 3 Cryptic give me pretty good all around coverage, and I think I should buff it out with 2 of the other options. Right now I'm testing Spell Snare (BGx was AssTrophy all over my orbs, Goyf is always a concern), but I can't decide what these 2 should be, or if I need more than 7 counterspells (up to 9?) My inclination would be mana leak or negate for these slots, perhaps 2 of each. Remand does a pretty good job at slowing them down while keeping our hand up.
I've considered Crabs and Scribes but have found them to just be a lightning rod for removal. I had my most success with my UB deck around AER release (ie: Fatal Push) by having no creatures in the main, leaves a lot of removal stranded. I've effectively used these slots to put the control package in.
One thing I've dropped is the Ensnaring Bridge - it doesn't play nicely with countermagic. Just had a thought that since I'm in white I could stick Ghostly Prison in to fill that role if need be.
I'm unsure if I need 2 Supreme Verdict AND 2 Settle the Wreckage in the main. I see it as Path handling big threats, and this handling go-wide strategies, which would otherwise be a tough matchup. Perhaps could slot 2 out for 2 Ghostly Prisons as anti go-wide.
Anyways, the list is not as out-there as you would think, it's still almost the same core mill package but with countermagic instead of creatures. White just complements the control strategy better than black does, in Modern at least. You could play similarly with a UB Control mill list, but UB control is more empty-handed, protect your threat on the board and tempo them down. But we're not really deploying a threat here, so it's not really ideal here. Being able to make your opponent tap mana into your Orbs, or holding up a Cryptic or Logic Knot (or Mission Briefing) to catch-all in the later game (where we are aiming to play to anyway) is a great place to be in my experience.
My sideboard has more than 15 cards since I'm still testing it out. A couple not mentioned that I've also considered: Spellskie, Leyline of Sanctity, Detention Sphere, Spreading Seas, Wall of Denial, Vapor Snag, Timely Reinforcements, Set Adrift