That is true, but I really think the difference in scale should be accounted for.
For example, just comparing Polymorph and Chiling Gaze highlights the differences in scale fairly well.
Polymorph can severely debilitate a single target unless it passes a single saving through.
Chilling Gaze debilitates targets even more (because unlike Polymorph, you can actually kill the targets without cancelling the effect), effects everything that is within, or will enter a 30' radius of you for 1 minute, and succeeding in the saving throw will only protect them for a single round (not forever as is the case with Polymorph).
At level 6, Chilling Gaze is dozens of times more powerful than the level 9 True Polymorph when used in this way. Although in True Polymorph's defense, it has some utility uses as well.
They are both powerful, and some might say overpowered; but Chilling Gaze is committing the far greater crime in that respect.
Just like any gaze attack, creatures can close their eyes to be immune, and it only works at the start of their turn so they can kite you. Granted, the duration certainly is not within the 5E idiom (2d4 days of paralysis?), but the 5E PHB itself has other spells that also don't fit the general 5E paradigm: consider unusual the 5th level Awaken spell in both enhancing and charming a beast for a whole month with no concentration requirement. Look at how broken the PHB's Animal Shapes is for dive-bombing an enemy with dozens of rhinoceroses at once (e.g. turn two dozen friendly mice into eagles, have them all fly to the target and then turn into rhinos); look at how Wall of Force doesn't even grant a save; look at how Aura of Vitality outclasses any other healing spell of comparable level; look at how True Polymorph can create permanent treasure and permanent Clay Golems and permanent Nycaloths that you can bind with Planar Binding; look at how Animate Dead breaks concentration economy and action economy into little tiny pieces.
I don't buy the claim that Chilling Gaze is more of an outlier than any spell in the PHB. (Hello, Simulacrum + Wish?!?)
But let's say for the sake of argument that Chilling Gaze is more powerful than anything in the PHB. So don't let your players research Chilling Gaze, or assign it a high DC to make them work for it! The fact that you may never hand out Chilling Gaze in treasure, or have any of your players research it, does not in any way detract from the awesomeness that is Spiteful Images! (It's a countermeasure against Mirror Image that makes the mirror images attack their own caster. Try and tell me that doesn't give you awesome ideas for all kinds of illusion-twisting spells!) And the very next spell in the list is Spyworm, a 3rd level druid spell that implants a parasite that lets you hear (for 8 hours) what the creature hears.
Every time I peruse the Book of Lost Spells I come away with awesome DMing ideas. To me that makes the product the best 5E product I have yet purchased.
Edit: BTW, on the subject of Polymorph, tell me another 4th level spell that can give another PC 230 extra HP (CR 13 Spinosaurus Rex, from Tome of Beasts) and 80ish DPR whenever you cast it. That spell is broken. Broken in a fun way, it's true, and as a DM I wouldn't ban it or change it--but if Chilling Gaze were in the PHB and Polymorph were in the BoLS, you might be criticizing Polymorph as the worse offender, and rightfully so.