The class series, when at its highest stage of Beast Lord and compared to the other series at their highest respective stages, has high HP, good Attack, and stronger Defense, but very low Agility (tying with the Angel and being higher only than the Hero) and the lowest PP rating of all classes. The series has a low Luck rating, tying with the Master and being higher only than the Chaos Lord. Noteworthy Psynergy from an endgame perspective includes Phoenix (equivalent to Revive), Manticore (exactly in between Cool Aura and Pure Wish), Ghost Soldier, Whiplash, Impact/High Impact, and Cure Poison, and the heaviest area-of-effect Psynergy it has is technically Fire Dragon. Average stats from an endgame perspective are 132% (144% if you don't factor in Luck).
The Tamer Class series is unique in that nearly all the Psynergy will change upon a change in class within this series. Thus, it is more convenient to think of the Psynergy in sets rather than series. However, all four classes in this series learn the Impact Psynergy series, Cure Poison, Restore, and Whiplash.
With all steps of the series, ranged Psynergy will do noticeably less damage to secondary targets than comparable Psynergy of most other classes, much like the Monster Skills used by the summoned monsters.
The Tamer is most likely the least useful of the item-dependent classes. Its extremely one-sided stat distribution implies the Tamer to be a tough, physical fighter. It somewhat accomplishes this, as the series has high HP with decent Attack and Defense. The Tamer has several additive damage or multiplicative damage Psynergies that take advantage of this. It also learns Cure Poison and the Impact series, which is always nice to have around.
However, the Tamer has extreme shortcomings that must be considered. Most of the Tamer's abilities are very underwhelming even at their highest forms. The Salamander series and the Wild Wolf series deal too little damage to be consistently useful. The Tamer also has a wealth of healing Psynergies and some buffing Psynergy, but there is a huge reason why all of the above does not work with the Tamer. Its stats make for an awful support job, having terrible agility and the absolute worst PP in the game. Even the class's luck is low as well, tying with the Brute classes. Another inconvenience is that despite the abundance of healing abilities, none of them are even able to be used outside of battle. Something else that should be noted is that all of the Tamer's area damage Psynergies possess low damage distribution, meaning that collateral targets take much less damage from already weak attacks. There is some merit in keeping an Adept in the Beastkeeper class(see bottom).
However, like all Item Dependent classes, specializing an Adept into the Tamer Class will leave an awkward number of Djinn floating around, which can be used to your advantage if you read the explanation about the Beast class above. Using the duplication glitch at Trial Road to create multiple copies of the class-changing items or making use of the extra Djinn to create tri-elemental classes like the Jupiter Adept version of Ninja are other ways to alleviate this issue.
Overall, the Tamer is a candidate for one of the worst classes in the game. The class desperately lacks in PP and agility, two critical stats for a functioning class. Their Psynergy set of various healing and support hardly compliments their stats, making for a Psynergy caster with the worst kind of stat distribution. Bar Fire Dragon and Ghost Soldier, most of the Tamer's abilities deal very underwhelming damage even from the get go. There are much better classes for an Adept to be in and a much better use for an inventory space. The White Mage and Hermit outperform it in support, while the Brute and Ninja classes have much better physical combat. The Tamer class is very unique, but it comes to bite back.
Very specific information about the Beastkeeper class: Edit
The Gryphon ability is the only one that can double the attack of a character when performed. On the top of that, the multiple healing psynergy Elder Wood can be a much better Wish Well for a lesser cost, thanks to the Triton's Ward. You won't regret using the Blue dragon at all for a multi-area effect, instead of the fire Dragon due to the enemies their lower mercury resistance in general.