Battle Pets, everyone loves to hate on 'em, but they do so for the wrong reasons.
From a design perspective, the idea of having a unit which is not controlled by a player is an interesting one. By giving them predictable movements and attack targets, it allows for players with greater understanding of positioning to anticipate targets, and govern where the battle pet’s focus will be.
So while normal units have “attacker advantage”, in that the player who controls the unit can decide where they move and attack, Battle Pets have “defender advantage”. Given that you have limited control over these units, they are given a slight buff to their base attack and health.
Now, as I said in the title, this is not why battle pets are weak. Lack of controlling them, while perceived as a weakness, is not their biggest flaw.
Buff Spells & Battle Pets
All the factions have various spells which provide an increase in attack and health of a unit, in exchange for a card spent, and a few extra crystals. Here is where the battle pets fail miserably. You simply cannot effectively use these cards with a battle pet.
Because the battle pet attacks at the start of your turn, before you have a chance to make any plays, they require that you play the spell to increase their stat line the turn before. This leads to potential blowouts by your opponent.
When you are using spells that increase a minions attack, you generally play the minion on one turn, positioning it to be close enough to the battle, but safe enough to survive to a future turn. Then on the subsequent turn, you play your spell to increase its attack, so that it becomes more effective. If you were to play your buff spell on the same turn as you play the unit, it presents a scenario where you have spent two cards and a decent amount of crystals, and put them all into a single square on the board.
Your opponent then has the opportunity to respond. Playing a removal card that is of lower cost than your minion + spell combined, gaining him both tempo (since he spends fewer crystals to remove your minion, than you spent on the minion plus buff spell combined), and card advantage (his one removal spell card, to your two cards, minion plus spell). Alternatively, your opponent can just dispel your buff effect, which negates the spells bonus, and any innate abilities your minion might have had.
Because of this, Battle Pets can never compete with regular minions. Since they always attack at the start of your turn, creating an anti-synergy with Buff Spells, which are a core component of almost every deck. Thus, the reason they are inherently weak cards, compared to any alternative choice at the same crystal cost.
Hopefully this has helped illuminate your understanding of battle pets. Allowing you to better discern if you should pick them for inclusion in your gauntlet decks. (I.E. Lots of buff spells; don’t pick battle pets! No buff spell; get that battle pet stat bonus!)