To add to what Celebi said:
All LEGENDs are really good, but they all have costly attacks with major drawbacks and (aside from Lugia) have no real way of accelerating that cost. SEL is playable in google only because it hits for weakness vs fire and steel types and doesn't lose energy in the process (using the first attack), and RDL is only played as a win-more tech in MagneBoar/ReshiBoar because only Emboar can reliably reload his attack.
Unfortunately, there just isn't any way to accelerate most energy types in this format (outside Electrode), and that's the biggest problem (IMO) with playing LEGENDs.
To make matters worse, no cards outside LEGEND Box (which is unplayable except vs Durant maybe) and Indigo Plateau were printed to support them. It's as if TPC thought them up, but then completely forgot to implement them effectively. Even something like a Master Ball that searched out both halves of a LEGEND, with some drawback like discarding/LZing a Pokémon, would have drastically increased their playability. Alas, they decided super tough, super strong Pokémon with very bad downsides weren't healthy for the game, so they opted for super tough, super strong Pokémon with barely any downside instead (Looking at you, Mewtwo EX)
/rant