Legend Tenacity vs Unflinching (Jungler)

I see many tanky Junglers who give up Alacrity and pick Legend Tenacity if enemy team has CC. But i rarely see people picking Unflinching instead.

I mean, is it that bad? It can give you up to 32% tenacity if you have Flash on CD and use your last Smite in fight. That is more often the case than you think. But 23,5% almost every time. From start on!

Both available 0% One activated, other available 23.5% One on cooldown 10% Second activated, other on cooldown 32% Two on cooldown 20%

Also i think Alacrity is way better for tanky junglers than that bit extra HP (around 150+) in lategame from Overgrowth because you have a lot HP anyway.

Also sometimes i see people taking both + Merc boots. Isn't that rather unfavorable because you weaken the runes because of the non-additive stacking?

Mercs: 30% Legend Tenacity: 30% Unflinching max: 32%

but all together "just" 67% instead of 92%

When i pick just Mercs and Unflinching i still get 52% max. So adding Legend Tenacity would give my just 15% extra Tenacity instead of 30% Why should i pick a "half" rune?

What do you think?