Are there any cons to mega evolving?
Short answer: Yes
Long answer: It depends. Mega Evolutions arent 'better' (for the most part, I'll come back to this). They are just another way of building a Pokemon, similar to if you built a Pokemon holding a Choice Item, or Life Orb etc.
Take Gengar for example. Standard Gengar is still a powerful force to be reckoned with, and its Ability Levitate makes it immune to Ground Type Attacks.
When Mega Evolving, Gengar changes its Ability to Shadow Tag, making it impossible for the other Pokemon to switch out (bar switching moves like Volt Switch). But this also means that it regains its weakness to Ground.
Now, if we simply compare stats, Mega Gengar has an extra 100 base stat total over its Normal form, however most of this is invested in its already high Special Attack, meaning even neutral attacks that land on Mega Gengar are going to leave a large dent in its mediocre HP. Its stat distributions are:
Gengar (Total: 500)
- HP: 60
- Attack: 65
- Defense: 60
- Sp.Atk: 130
- Sp.Def: 75
- Speed: 110
Mega Gengar (Total: 600)
- HP: 60
- Attack: 65
- Defense: 80
- Sp.Atk: 170
- Sp.Def: 95
- Speed: 130
As such, the biggest con of Mega Evolving Gengar is the loss of Levitate. Ground is a very common attacking type, with Earthquake pretty much being a staple move on anything that can learn it. Coupled with the little to no investment in HP and Defense, Mega Gengar becomes an even frailer 'Glass Cannon' than its Normal form.
But what about the Mega Evolutions that change their types?
Mega Charizard X is finally a real Fire/Dragon (Dragonite can stop laughing at him).
Charizard: (Total: 534)
- HP: 78
- Attack: 84
- Defense: 78
- Sp.Atk: 109
- Sp.Def: 85
- Speed: 100
Mega Charizard X (Total: 634)
- HP: 78
- Attack: 130
- Defense: 111
- Sp.Atk: 130
- Sp.Def: 85
- Speed: 100
It gets a large boost to Defense, its (Physical) Attack is raised to be on par with its Special Attack, and its Ability changes to Tough Claws, further boosting its Physical Attack. As such, builds for Mega Charizard X tend to have powerful Fire and Dragon moves (as opposed to Flying), and with its higher Attack stat Mega Charizard X can run either a pure Physical or Mixed attack set.
But unfortunately with the introduction of the Fairy type, Dragon isn't the overpowered type it used to be. Plus, Mega Charizard X loses its Flying type, regaining Fire's weakness to Ground.
The con for Mega Evolving in this case, is again going up against the common Ground types, and the new powerful Fairy type moves which pop up more often than you think.
Side note: I've surprised many Dragons with Dazzling Gleam on Gengar ;)
So, in summary:
Are Mega Evolutions 'better'? Sure, if you only look at the stat boosts, most if not all gain a buff to their stats, but this usually comes at a cost of opening up different weaknesses to exploit.
Does every team need a Mega Evolution? No. Most teams will work just fine without one. At the same time, most teams could incorporate a Mega Evolution and it would work just as well. Think of Mega Evolutions as just another way of building a Pokemon.
Note: Sometimes it pays to play the psychological game. If they think you're running a Mega Evolution, they may take particular steps to counter it, and you can catch them off guard. Sometimes it's even worth it to hold off Mega Evolving for a turn or two: for example, predicting an Earthquake with Gengar.
Just wanna point out that some pokemon also lose stats when they mega evolve. Take Garchomp for example. He gains a great boost to Sp. Def and a fair boost to Att and Def but loses his well renowned speed, becoming more of a tank than a fast hitter. Also, with regards to Lucario, (he has been one of my favourite Pokemon since gen 4) he is a very poweful Special sweeper in his normal form with moves like Aura Sphere which never misses, Nasty Plot and Vaccum Wave for type boosted priority. However, when he megas he becomes a much more powerful Physical sweeper with a boost to Att and Spd and powerful combos of Adaptability and Close Combat/Iron Tail. Another of my favourites, Absol gains some Sp. Att which is wasted on him and his new abilitu Magic Bounce only works if the opponent uses a status move on him and after being used once wont usually work again (unless the oponent is a retard). Compared to Absols other set up of Super Luck, Scope Lens and Night Slash for guarenteed critical hits with Absols very high Att and fair Spd, its quite easy to see the normal is better than the mega. All in all, it depends on the Pokemon in question. Some, such as Tyranitar are better of in thier normal form with a useful item (Assault vest for a Physical Tyranitar) but some such as Scizor and Mewtwo are boosted so much by them that to not mega evolve them would be a waste. Last thing I want to point out, Mega Mewtwo (both x and y) have the highest base stat total out of any Pokemon whatsoever, so its a very good idea to mega them.