I keep checking these threads hoping someone has had an epiphany, but it's mostly the same stuff.
He's really good at stealing enemy jungles if he has smite, but as a support he can't really afford to take it. When playing as support you need a good team composition to compensate and get bonuses from his kit that others wouldn't. He needs to build tanky support and nothing else.
On the flipside, his kit has only one main combo revolving around Daisy (R) basically tagging Daisy with his shield (E) and then Q'ing an enemy to initiate with her. (She jumps to them when they're snared and gets about 2 of her 3 hits off, the shield pops, they're slowed, she gets her third hit, anyone caught is knocked up.) Basically a solid initiation.
BUT... he has no follow-through dps except his W. Which means being in sub-ADC ranged range. His on-hits are really quite strong for what they are, but as an AP support champ, AS is not a very desirable stat. However Nashor's compensates for his entire build on a few levels, bumping up his AS so his W's on-hits are meaningful in frequency, cooldowns so Daisy can be around more and the AP on-hit to basically crank up that W on-hit from autos. Unfortunately, Nashor's is still too pricey to build reliably and there are better items you need before you actually can afford to diverge and build it.
Playing Dominion definitely filled in gaps I had about how to play him in teamfights, but I wouldn't advise it since it's very detrimental to your team in the bigger picture. As you build defense, you are a bit tankier than you'd think and can play an in-and-out sort of combat where you're poking with your W's on-hit between bits... running out of a bush for a few auto-attacks and generally you need to keep moving chaotically. You're basically trying to get an opening to nail an enemy champ and overall take out the enemy dps, or at least create an opening for a good initiator. If you don't have an initiator, you're it and not a very good one, so Daisy has to go first generally.
AS STATED a few times and repeatedly, the big draw to having him on your team is spare jungle buffs for him. (Or for teammates if he's jungle.) If you utilize it correctly, both buffs play into his W's on-hit dps. This is really important to note, since again, it's his only real sustained dps in the bigger picture.
BIG DRAWBACKS:
Minions soak his Q. Ultimately, I think this is where his kit really flops. At first I thought it was one thing or another, but he relies on it so much that if literally anything else soaks it, his combo is pretty trashed and he definitely can't initiate.
Daisy's targeting is heavily user-end. She doesn't auto-focus any targets by default unless they're in the place you left her last you told her to move. which means you need to R an enemy champ and stay in range so she doesn't disengage. Her targeting is severely hinky in this respect.
Bushes are the enemy's friend too. Despite how much it plays havoc on an enemy, it only takes one Teemo or enemy that can abuse the brush in any manner to make them awful. It means that you need to spawn brush to cut line of sight from enemies chasing or in a safe place slighty behind you to abuse your W and give your team a minor respite and retreat. It means you have to be careful not to plan ahead like the tips suggest, because an enemy can turn that into an ambush almost as well.