That was with rolling, you know - the standard system for character generation. About one in six characters will have a 16+ and a 17+ score in their stat line. Nearly one in two characters will have a 15+ and a 16+ (allowing 18 Dex, 16 Int, going to 20 Dex at level 4, for AC 21 / 26 with Shield.)How do they have a 20?
Here I was exaggerating for effect. The point is, Bladesinger is a full-caster. No loss of Wizard power. Straight Wizard levels is the bar for power in D&D. Even without having tank AC and decent melee damage (with Green-Flame Blade).Umm.... fly, invisibility, and polymorph are all Concentration spells. You can't cast more than one of those at a time. You can be invisible or you can be hiding, but not both. And once you cast your spell you lose the other buff.
There aren't really three challenging combats per short rest, in any campaign I've yet seen.Not being up all the time is a HUGE catch. 2-3 combats per short rest is an average.
Which means that a third of the time it will be those two combats, a third of the time it might just be one combat between short rests, and a third of the time it could be four combats between short rests. And while the ones that count the character will have bladesinging up, that still means two combats where they're the party's tank and have an AC of 18. Which is still decent but by no means unhittable.
Huh? Fighter is capped at 21 unless they find magic armor. If Bladesinger finds magic Studded Leather, they switch to it. The odds of light magical armor are the same as those of heavy. If Bladesinger takes ASIs at 4th and 8th, they gain a point over the heaviest martial, even before casting Shield.That's also maximum AC. They can't ever find magic armour. Or a magic shield. There's maybe a single item that can boost their AC at that point. Meanwhile, monster accuracy just keeps increasing. While nothing is going to hit reliably at level 4, at level 8 or 10 that changes. The CR 5 hill giant is going to hit 30% of the time and makes two attacks. And the CR 9 fire giant also has two attacks and hits 45% of the time. And does 28 damage on a hit, which will serious wreck the day of a bladesinger with 50 hit points at level 8.
You do correctly identify that their weakness is hit points. They'll probably want their third highest stat to be on Constitution. Most Wizards I've so far seen, put their second highest stat on Constitution. The thing is that unlike the heavy Fighter, the Bladesinger can choose to take a caster role, against things that look likely to be able to hit them, and move to a melee role against other creatures (where GFB becomes better, with bounces). Casting is still the strongest strategy in D&D. Why should a caster be anywhere near a martial at melee, without sacrifice?