I think you are a bit off in the comparisons. For example, Dragonkin racial is easily juked. Then I just must say that the Undead racial is even easier to juke, since it is only one time and after that the pet is dead while the Dragonkin racial can be applied over and over.
Undead: But I do agree that the Undead racial is very strong, essentially a bonus turn. And if you just go head to head with another pet chances are that they will die on the same turn, but the undead pet will live for another turn and either get a head start against the next pet or if it was the last two pets actually win the fight. That said your opponent will likely know when the undead racial will be applied and you can't do much other than an attack. Since your opponent knows you can't be hurt they could take this turn to do something else instead, heal/avoid/stun/blind/set up a buff. Best counter to this racial though is a switch move, and that you actually kill the pet with the switch move so that the racial will be wasted in the back line.
Critter: Yes very useful, but still can get sleeped by Moths... must be a bug. But critters in general seem to see much less playtime than other pet families(perhaps except magic). This racial is good since you can switch in a critter if you see a delayed stun. But your opponent can also avoid this racial by not using stun/sleep/roots, but if your opponents plan needs these moves then the racial becomes much more valuable.
Flyer - Speed is great tactical advantage, and birds have this more than most. I believe this is a very balanced racial. Also useful with healing teams or like Pterrodax Hatchling, since you can heal back up to above 50% and get the bonus back.
Humanoid - Amongst the humanoids this is very hit or miss. Some, like you said, heal from back line and others like Anubisath Idol has such a high health pool that the heals are bigger and coupled with sandstorm shield makes them even more durable. Other humanoids does not nearly get any benefit at all from this racial, heals are just too small.
Mechanical - Yes similar to the undead racial but it has both advantages and disadvantages. Disadvantages: If your opponent has multi hit move or you got dots on you you might not even get to use that extra turn since you don't get immune to attacks like the undead. Advantages: If you plan around it and get some repair or wish in you essentially get a second life, and you can back line the pet and perhaps heal again. This the undead raical can't do.
Dragonkin - In it's current incarnation I find this very useful. After you get your opponent to below 50% it can be pretty much game over if they don't juke it. That being said it does not seem to be able to be applied two turns in a row, luckily. I just don't think the ability is very clear when it activates, it seems that as soon as the opponent is below 50% any attack you do will activate the racial. It also activates when pets in back line gets hurt, so if you have dots or cyclone or other AoE attack that can hurt a back line pet that is below 50% the racial will activate even though the active pet might be 100%.
Aquatic - Good balanced racial, but since 5.4 I think I have faced a total of 1 DoT heavy team, so right now it is mostly used as a counter to Valks(which on the other hand there is an abundance of).
Beast - Yes, 25% is a bit of a risk. But you can increase this with abilities like Crouch. I love this with Scalded Basilisk Hatchling that can really put the hurt to your opponent after 25%. The S/S death adder attacks are actually pitifully weak until it hits 25%.
Other beasts with go-first moves also benefits nicely from the racial, direhorns/dimetardon.
Magic - I never get any use of this in PvP. No sane opponent would use a 100-150% damage booster vs a magic pet. I only find it useful when taking a level 1 up against a trainer to level it. The level 1 will take like 40 dmg from a lvl 25 pet.
Elemental - Yea pretty much only No Blindness from Darkness is useful.