This build will make you less usful to your adc during lane phase. its very situational.
mainly to be used if you're lacking a frontline on your team, and you don't actually want to play a frontliner. and they have heavy burst that would delete you if you were focused.

theres been plenty of times i first picked janna and the team refused to pick a front line champion. And this scenario seems so common for me.
the enemy picks a jung like reng or yi, that 1shots me every time i try to ward because we have no pressure, the adc i get is usually something like an ashe that builds trinity force first, and manages to do 400proc dmg 20mins later. "i'll never understand the tf build" that always seems to do 15k total dmg out of a 40min game, so no matter how much shielding,healing,cc,warding you do everyteam fight is a loss and you can't make any impact... even supporting other lanes who also build glasscannon/lethality with less then 15k total dmg. point is utility is worthless when your team has no followup.

And with the standard janna build, when behind and lack of a actual team. you cant walk in the middle of a fight and heal your whole team, you gotta safely postion. and like i said its hard to ward. with this build you have less shielding and maybe like 20-30% less healing with your ult, radiant virtue makes up for some of it, but still less. This build makes you tanky enough to safely ward, it lets you frontline so your team can position better, and it lets you be in the middle of fights so you can ult your whole team. its like milos ult with the added knockback and better ap scaling. and if you can heal 4team mates instead of 1-2 thats going to give a massive boost to how impactful you are. the goal should always be to try to heal your entire team. this seems impossible to do playing with the standard build because you're immobile as it channels and die to fast.

Lastly i want to pointout in low elo, emerald or lower, you always see someone saying "this is low elo, frontline doesn't matter" like you can easily overcome it with the skill difference. The thing is you're playing support, you can't snowball and 1v5. and the people saying this are in the same elo as the other team, there is no skill difference. it's a diceroll to see who has a good or bad game. As a support never let your team have no frontline, or be full ad or ap. which is the point of this build, if you have to or want to first pick, and you get screwed by your team, its better to have an option like this. my advice is if you first pick as a support, or even in general it's best to play a veritile support like twitch that can do ap or ad plus true dmg and slow, or senna that can tank,cc and do dmg. supports like yummi, leona, soraka, bard who are strong at doing 1 specific thing arent reliable enough to change your strategy if needed.