That's your subjective opinion.
Objectively power creep in effect has obsolete most of the old clases and archetypes.
It's not unplayable but weak. Every martial class without masteries cone to mind.
As I think I said earlier in this post, if you are mixing old and new you should follow WOTC suggestions and replace anything that is in 2024 with the new version. So the only "old class" you would be using if you did this is artificer as all the others are reprinted. You can use old subclasses with the new classes pretty easily though.
In the game so far I have played or am playing the following:
PC1: Kobold (old) - Whispers (old) Bard (new)/Swashbuckler (old) Rogue (new)/Fey(new), Warlock(new)/Ranger (new) with Shadow Touched feat (new), Magic Initiate Feat (new) with Cloak of Flies (old) and other new invocations. Had old spells including Cause Fear, Booming Blade, Green Flame Blade and Enemies Abound
PC2: Halfling (new) - Fey(new) Warlock(new)/Arcane Trickster (new) Rogue(new)/Bard(new)/Paladin(new) with 2nd Chance feat (old), Magic Initiate Feat (new), Warcaster Feat (new), musician (new) with old spells: Cause Fear, Green Flame Blade, Booming Blade
PC3: Human (new) - Scout (old) Rogue (new)/Drunken (old) Monk (new)/Wizard (new) with Tough feat (new), Grappler feat(new).
PC4: Dragonborn (new) - Fey Wanderer (new) Ranger (new)/Glamour (new) Bard (new)/Monk(new) with Dragonfear Feat (old), Shadow Touched Feat (new)
PC5: Infernal Tiefling (new) - Eldritch Knight (new) Fighter (new)/Warlock (new) with Agonizing Green-Flame Blade (new evocation with old spell), Booming Blade (old), Truestrike (new), Pact of Blade (new), Pact of tome with Shilleleagh (new), Find Familiar (new), Create Bonfire (old), magic initiate feat (new), Flames of Phelethegos feat (old), Warcaster feat (new), Shadow Touched feat (new).
All of those PCs combine old and new pretty seamlessly. The key here though is anything reprinted in 2024 you use the new version. If you are not doing that there are going to be problems.