First and foremost, legion is a very cool new mechanic that allows you to have two units on your vanguard circle! This means you will have two vanguards! Amazing! (haha)
I think Legion is a very cool idea. It certainly beats Limit Break as you don't have to wait for four damage to actually do stuff. I never really liked the idea of Limit Break. As my main clan is Oracle Think Tank, I always felt that it was way too slow for what it did. You gain as much pluses from Limit Break abilities as you would from non-Limit Breaks.
The only restriction is that you can only Legion if your opponent has a grade 3 or greater vanguard, and you need at least four cards in your drop zone. That is not much of a downside. Legion allows you to recycle triggers! This could be very important especially if they are heal triggers that got into your hand during early of the game. Furthermore, when you are in Legion, your vanguard attacks for 20000 without boosts! Talk about going beyond the limits, Legion can do it better!
That is the problem. Legion does make Limit Breaks look bad, maybe bar some very good ones. Also since Legion is a lot faster than Limit Break, Break Rides could be too slow. It gets worse when your opponent tries to trap you at three damage.
However, like any decks, there are ways to play against Legion decks. It is not easy but you would have to be able to match up to their attack speed. One of the most powerful strategy using Legion is to get your opponent to high enough damage so that your Legion skill would have that much more pressure. As a Limit Break player, you would need to be able to do the same to your opponent while also defending. This eats up resources for both players as they use cards in their hand to attack and defend, and would make it more of a fair game for both players.
This means while Limit Break has more importance on damage managing, Legion focuses more on resource management. For Legion decks, you will need to get four cards into your drop zone as soon as possible. It would also be better if the cards are triggers. Though it might be easier for some clans, it is not always easy to get the right amount of cards, and the correct type of cards into your drop zone. Sometimes you might even have to sacrifice some rear-guards. This is where Quintet Wall shines best. It allows you to guard early game with ease while getting cards into your drop zone at the same time.
In conclusion, Legion is very powerful. Constant 20k attack makes a lot of 10k vanguard based build to be very weak. Aside from Legion skills which are on par with Limit Break, some units even have very powerful alternate skills. Legions also brought notice to some older cards like Cat Butler and Weather Forecaster, Miss Mist. There is so much more to write on Legion but this is my take on the subject for now.
CoCo 4 Lyf