Fervor of Battle vs Grasp of the Undying

That's how ridiculous it is

well you see, everyone thinks you choose the mastery-tree depending on the best keystone but thats actually not the case. the reason everyone takes thunderlords is not because its the best keystone, its because its in the best tree.

even for tanks, precision is just too strong so thunderlords is the natural choice

{quoted}

Thunderlord for pure tanks?

nah i'm not sure about that but it wouldn't make any sense on a tank as i said "almost"

So your process about what keystone mastery(or masteries) to pick should comprehend various steps

1. Choose the keystone mastery that seems the stronger for the champ

2. Look at the mastery tree the keystone is in, and do yourself this question "can I spend well my mastery points to fill the tree with useful things for my champ(s) before getting to keystone?". If you have to waste 5-6 or more points on useless things to get the "best" mastery on that tree, it is not worth at all, and to see if there are things useful or not on the previous tiers depends on how do you play

3. If the answer to the previous question is yes, heck, well done, otherwise you rinse and repeat the same process for the best next keystone mastery

...

5. You can even consider taking no keystone masteries if You can never been satisfied with the answer at point 2 and you see there is no valuable keystone mastery left in the process

You should think about how you play and do the process of thinking yourself, imho

The new keystone masteries aren't "universal", but they are situational/circumstantial about your game style, and so the rest of the trees that are fairly mixed with various things, especially the cunning tree, so you should really try to figure it out how you play and adjust the whole mastery page accordingly