At Tier 2, Level 2 spells start to matter less. You can be upcasting that Spiritual Weapon into a 4th level slot for double dice on each hit. Or upcasting that Maximized Thunderwave, Shatter, or Call Lightning into a 4th or 5th level slot for a flat 40 or 48/50 damage. While Radiance of the Dawn is still doing 2d10 + 7/8/9.
You are correct that Thunderwave/Shatter are CON saves, which are less ideal in some situations. But Call Lightning is a DEX save. And it does its damage round after round, with one casting. So while you may be saving the equivalent of 2nd level slots with Radiance of the Dawn, you're saving 3rd (or higher) slots with Call Lightning. And it uses your Action on subsequent turns to call down additional bolts, which leaves your Bonus Actions free, allowing it to still be used in conjunction with Spiritual Weapon, Healing Word, etc.
Radiance of the Dawn is a fine ability, for the reasons you've pointed out. But look at it this way, apples to apples: It's a less powerful Channel Divinity ability than maximizing your thunder/lightning damage, especially at higher levels (5+). And Fireball is a less powerful 3rd level domain spell than Call Lightning. Faerie Fire is a great spell, when your Concentration isn't in use elsewhere, but not enough to make up the difference. So the Tempest Cleric pulls well ahead as far as blasters go, in my book.