Heath is imo the best flier in the game behind Florina by a pretty big margin. He totally blows Fiora (and Farina) out of the water offensively upon promotion thanks to his much much better strength, and the speed gap isn't even really existant thanks to his much better con (and Heath actually only has 5% less speed growth than Fiora and the same speed growth as Farina). A 12/1 Heath has 12 AS with a Javelin, a 14/1 Fiora has 11-12 herself and 3 less attack (and this will only continue to grow since Heath's growth is significantly higher).
His res is a lot worse, but promoted enemies, even in HHM are awfully weak. Heath with a pure water or barrier staff isnt going to die anytime soon to them. His physical durability is a real advantage, Fiora''s evade come mid-lategame (lets say she's 14/8 to Heath's 12/8), is 6 better than Heath's before you account for attack speed loss which Fiora definately will have. So at the very least, he draws even or even beats her in that catergory if she uses a weapon heavier than 9 WT (Javelins, Axereavers....). At the same level, Heath has ~10 more HP and 5 more defence,. Against enemies with 25 attack, Heath eats 2 more hits, against enemies with 20, he eats 5. It's a pretty huge gap. Offensively Fiora loses every metric possible.
The last advantage Heath has is minor, but he can rescue Hector once Hector promotes, since he has 1 more aid than Fiora and Farina. It's minor, but having a flier to haul around your 5 mov lord is definitely a boon.
Fiora's advantages (an extra 3.5ish chapters, only one of is is flier friendly), and much higher res aren't really comparable to all of Heath's advantages. Even lategame Heath will outperform Florina offensively unless you played Lyn mode. Florina is definately still better due to a huge availability lead, but their performances after Heath's promotion are very similar and probably even in Heath's favour lategame.