Emolga is a situational pokemon searcher, but so is every part of the Ball Engine. If one was "clearly" superior, then the others would be moot to ever run.
Let's take a look at each part of the Pokemon Search Engine and determine it's flaws and props:
Emolga:
Props:
-Searches for 2 pokemon with one energy...effectively 2 cards for 2 Pokemon
-Free Retreat - Having a free retreater to promote is nice when you're letting your next draw decide your plan, or vital if playing Eelektrik
-70HP - Is not donked by many deck's donking power (Only Tornadus can boast this, but as he's known for donking, and how he has to expend a Pluspower to do so shows Emolga's relative bulk)
-Is Searchable - This is big, you can turn your Level Ball and attack for the turn to instead search out 2 Basics...maybe even ones unsearchable by other means (Like Zekrom and co)
-The basics it pulls can be evolved next turn - This is why it's so popular in Stage 2 decks, and I predict will be popular in Eelektrik decks.
-Reusable w/o support - Trainer Cards can't be used again without the help of Junk Arm (Out of rotation) or Sableye (Also requires an attack), whereas Emolga can sing the second verse of the song without any further need (except to be alive), Making Emolga one of the more efficient searchers (except Level Ball for dragon decks (Read: Gabite)).
Slops:
-Takes your attack for the turn - At the beginning of the game, this isn't that bad (except against a donking pokemon), but if you're forced to use this mid-game or late-game, your opponent's taking a Prize.
-Takes an energy - He takes one of your deck's energies to actually search for the pokemon. With his free retreat, the energy is not leaving Emolga except by death, so expect to play with one less energy in your deck (something big for decks that run low energy like Empoleon).
-70HP - This is both a blessing and a curse. Blessing in the sense he's hard to donk, curse in the sense he's now catcher-bait. You either have to waste a SSU to prevent this, or accept after he does his job he's a sitting duck (Squirrel, whatever)
-Being a pokemon - This means he's taking up a bench space after he fulfills his job, and this can hinder decks that really need all the bench space they can get (Like Eelektrik). Again, SSU is an option, but I'd relegate its use to other pokemon.
-Searches only Basics - This means it can't shift roles as needed. Where as the trainer searches can search evolutions if needed, Emolga's limited to basic pokemon.
Level Ball:
Props:
-Trainer Card - This means it can be used without taking up room on the playmat except in the discard pile to search your deck. This is something it has over Emolga.
-No Side-effects - This is why Level and Heavy Ball are so popular for searching pokemon if applicable: except the card, you lose nothing. A true 1-1 trade.
Slops:
-Limited to <100HP - This means that decks that run higher HP pokemon than this (Read: Every Deck bar Ninetales/Amoonguss) will be forced to run other forms of Pokemon search.
-Trainer Card - Not as much of a problem as it was when Vileplume was in rotation, but against Zebstrika and Gothitelle, its usefulness just becomes junk.
Heavy Ball:
Props:
-Trainer Card - See Level Ball for this
-No Drawback - Just like Level Ball, there's no cost of using this card as with say Emolga (Energy) or Ultra Ball (2 Discard)
Slops:
-Searches for fewer pokemon - 3 Retreat is pretty high, something most decks are hesitant to run because of catcher. This means Heavy Ball is delegated to pull out certain pokemon otherwise not needed (Klink-line, Entei EX in Quads, Terrakion, Hydreigon, Registeel, etc). Retreat is a turn-off on pokemon, meaning the more heavy ball applicable you are, the more catcher-bait you are.
-Trainer Card - Again, see level ball for this
Ultra Ball:
Props:
-Searches ALL pokemon - This makes Ultra ball the most universal searching card in the game. at the cost of Ultra ball and 2 cards, you can pull that pokemon the moment you need them.
-Discard engine - This part of the card is beautiful in decks that play from the discard, like Dark variants and Eelektrik. Getting the cards in the discard earlier gives the deck a faster set-up...and still nabs a vital pokemon for the process!
-Trainer Card - See Level Ball for this one
Slops:
-Discards - I said this was a prop, right? Well, in decks that don't like having things in the discard (Read: Everything except Discard abusers), Ultra ball can force unnecessary discarding of potential important cards. Also in discard abusing decks, there's teh chance you might not have the cards you actually wish to discard in-hand, meaning you're discarding sub-optimal cards to get one pokemon.
-The Card Trade - Besides Pokemon Communication, Ultra Ball is the only card that doesn't get the same amount of cards it uses. At the cost of 3 cards, you're searching one card. Everything else is a 1-1 trade (Emolga 2-2), making Ultra Ball not the most efficient.
-Trainer Card - See Level Ball
-Inferiority - In some cases, Ultra ball's the only thing that can search it from the deck. In most cases, Ultra ball's a failsafe to search for what you need when you need it. However, as it actually has a drawback to its use, using Ultra ball to search out Eelektrik just makes it a worse Level Ball for that use (Unless discarding Lightning energy, but I digress)
Pokemon Communication:
Props:
-Versatility - It searches for any pokemon in your deck. This makes it powerful alone, but with Level Ball/Heavy Ball can be used as a psuedo-Ultra Ball, if need be.
-Trainer Card - See Level Ball
-Other Use - Yes, like Ultra Ball, PComm actually has another use, albeit usually useless...It lets you put a card back in your deck, but then you can fail the search. You've thinned out your hand, and potentially beefed up your deck against a milling deck.
Slops:
-Requires another card - Remember how I said it and Ultra Ball were the only ones not to give a 1-1 trade? Well, PComm requires another card to be used up (But not discarded) to actually search. I'd say it's like a 1.5-1 Trade, as the pokemon used up isn't actually discarded. Still a slop when compared to the other cards.
Please, if I missed anything about the cards, don't go on a rant. Just post and I'll edit it in so new readers can find all this in one post...
But I hope I made my stance on searching clear...(I know I left out Gabite, he's specific to one type).