Sure, it looks like a lot. Until you put it in context.
To cast phantom steed, a wizard has to be 5th level. A sorcerer has to be 6th. A bard has to be 7th. For the sake of our example, we'll use wizard.
Let's assume you want to be as powerful a spellcaster as possible, while still taking the knight phantom PrC. By the time you're 20th level, you're likely a fighter 1/wizard 9/knight 10. You've got 1d10 (average 4.5, we'll say 5) + 9d4 (average 22.5, we'll say 23) + 10d8 (average 45) hp, for a rough total of 73. Your BAB is 15. You cast spells as an 18th-level caster. Your saves are Fort +9/Ref +6/Will +9. You can cast spells in light armor. You have a few nifty class abilities; they're cool, but they're not amazingly potent.
Now, let's compare you to a 20th-level cleric. The cleric has 20d8 hp (average 90), beating you by 17. The cleric has the same BAB you do. He's a 20th-level caster, not 18th. His saves are +12/+6/+12, putting him 3 ahead in Fort and Will, and equal to you in Reflex. He can cast spells in all armor, plus shields. He, too, has a few nifty class abilities (turn undead, two domain powers).
(Yes, I'm ignoring the fact that you get max HP at 1st level, because that's going to change based on which class you take first. But it's only going to shift the numbers by a couple of points at most; not enough to change the conclusion.)
Compare you to a 20th-level wizard. Sure, you've got him beat on BAB, HP, and one save. But you cast at two levels lower, your Will save isn't as good, and you're a whole mess of feats behind him.
The balance of a PrC isn't just in what it gives you; it's in how a total progression compares to other characters. The knight phantom sacrifices power for flexibility. Is it more flexible than a "normal" multiclass? Sure, but let's be honest. Some multiclass combinations--fighter/wizard among them--simply suck. Is it better than eldritch knight? Yeah, probably, but I'll happily argue (if you really want to) that EK is under-powered.
Is it overpowered? Unbalancing? Not even remotely. In fact, this is really a very cool PrC from both a play and design perspective, and I'd happily allow it not only in an Eberron campaign, but in most others.