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.
Statistically, the Tamer leaves a bit to be desired: Sub-par agility with the dubious misfortune of having a below-average PP pool and low Luck is not a good sign. However, the Beast Lord has a pretty good Attack stat, and also fortuitous is the fact that the mainline damage Psynergy abilities (namely, the beast summons) draw from the attack stat, so the PP pool might go a little farther than it would otherwise.
A notable thing to say about the Beastkeeper class is that Djinni-wise, it is interesting to keep the trainer in the latter class. The big advantage consists of having very stable and good psynergy, while being able to use multiple Djinn with a low stat reduction. The overall stat loss you suffer (compared to the Beast Lord) can be neglected, thanks to HP and Defense raising equipment, stat Boosters and a more stable movepool and statistics after or before using Djinni.
Very specific information about the Beastkeeper class:
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. The equipment improves Mercury Resistance and Psynergy Powers, making it overall perfect for long battles, especially if equiped with a crown of Glory. 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.
The Tamer classes themselves in general are noteworthy for their single target attack Psynergies, making them better for boss fights. In addition, the low agility can allow a Tamer to be used like a Paladin (heal during the end of the turn). Despite the low PP pool, the wide range of psynergy available to the Tamer makes it a very flexible class.
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.