Yanmega is a fairly nifty card. 90 HP is average for a Stage 1, +20 Weakness to Lightning might not be so good to have with Luxray around, -20 Resistance to Fighting is ok, and a free Retreat Cost is always welcomed. Sonic Wave offers some of the best spread damage out there; 10 to the active, 10 to all the benched, and a forced switch, all for 0 energy. Granted, the forced switch could potentially backfire at times, but it should be nice early game. Pursue and Turn is likely an attack you'll never use; 60 damage for 2 Grass and 2 Colorless Energy is horrible, even if it has the potential to do an additional 20 if the defending has at least 2 damage counters, and you have to switch Yanmega out (which could or could not be good). But it is Yanmega's only beefy attack, so if you have nothing else to throw energy onto, may as well stockpile some on Yanmega.
Modified - 7/10 (Good card, gets weighed down in the face of other spread damage Pokemon like Froslass or Bronzong, but can still hold its own)
Limited - 8/10 (You might be able to afford the time to power up Pursue and Turn here, and Sonic Wave is nice at nailing everything)
Unlimited - 1/10 (No real use here imo, spread decks are sort of meh)