The only restriction is it can only be played once per turn, and it places both in defense, but these seem distinctively less relevant as far as limitations to the card
This has led to card supply shortages, people fighting over new cards, thieves targeting stores, and prices for rare cards exploding online
Mega Evolution MEGA thrilling, MEGA powerful, MEGA fun
That matters because modern Pokemon has full-art cards, ultra rares, secret rares, and other foil-heavy treatments where calling something just holo does not really tell you enough

Mega Venusaur ex - 003/132 #3Double Rare $0.96 Snom #42Common $0.07 Numel #21Common $0.05 Volcanion #25Uncommon $0.02 Mega Manectric ex - 050/132 #50Double Rare $0.72 Heliolisk #53Common $0.06 Magnezone #47Uncommon $0.09 Exeggcute #4Common $0.04 Kyogre #34Rare $0.21 Pyroar #24Uncommon $0.04 Call Of Legends Theme Deck - "Retort" [Lucario] Flashfire Collector Pin 3 Pack Blister [Mega Lucario] Mega Lucario Collection Steam Siege Single Pack Blister [Lucario] $63.73 Crimson Invasion 3 Pack Blister [Lucario] $53.31 Lucario GX Box Knock Out Collection [Lucario, Bisharp, and Zoroark] $179.98 V Forces Tin [Lucario V] $51.17 Lucario #6Holo Rare $63.51 Lucario #2Rare $31.07 Lucario #61Uncommon $7.55 Lucario #2Rare $23.63 Lucario #53Uncommon $6.54 Lucario #12Promo $280.48 Lucario #19Rare $5.07 Lucario #14Holo Rare $13.83 Lucario #64Holo Rare $11.32 Lucario #80Holo Rare $20.57