Why Ezreal/Janna is a bad lane

Janna's Strengths:
--Shield (sustain IF AND ONLY IF the ADC is trading or getting poked)
--Damage (through AD bonus to ADC, multiplied by ADC having good AD-scaling skills or trading multiple autoattacks)
--Peel (useful if ADC needs peel)
--Chasing (useful if ADC can just chase down enemy botlane)

Janna's Weaknesses:
--Can't trade herself (so needs a long-range ADC to poke; otherwise ADC will soak too much damage if goes to trade)
--Not great in an allin at early levels (AD bonus doesn't last for that long on shield, shield not that big yet, doesn't have good damage nuke of her own)
--Very squishy herself (can't act as a damage soak for ADC)
--No waveclear

Ezreal's strengths:
--Good poke in lane (gets to stay back and not trade)
--Mobility (doesn't need much peel and also can dodge poke)
--Good level 2 allin

Ezreal's weaknesses:
--Not great in longterm trades (passive isn't that good compared to other steroids)
--Subpar waveclear pre-6
--Weak AA poke (due to standard range)

Basically, Janna's AD shield is a HUGE part of her kit since she has no damage nuke of her own.  But Ezreal can't really make use of that shield at all!  Janna also provides fantastic peel to someone like Caitlyn or Vayne, which is something that Ezreal doesn't need.  Ezreal prefers to stand back and poke, and he appreciates true sustain (like what he would get from Nami or Sona).  Janna's passive-ish give-AD-to-the-ADC style of play doesn't synergize with Ezreal at ALL, making the lane extremely weak.