First things first, I haven't played a game with biests yet, so this is just a preconception.
The level 2 biest class seems hardly worth it. You pay double the base species cost up front for an extra attack. With biests already having a base cost this gets expensive quickly. Secondly they don't have acced to wildling which was discussed in another thread as being intentional. They can buy a characteristic increase for 10 points. So compared to the level 1 class you get: +1 attack, +1 max properties, +1 max class traits. You pay: +1x species cost, +6 (poinst difference) for a characteristic increase
And also the option to pay 4 points for a situational bonus.
Again, I am not in a position to judge how good or bad it actually is, but it feels like you are paying a hefty premium for a fairly meager benefit in the form of room to grow. Which, in terms of traits will only make the model more expensive. If I can field almost 2 level 1 models for each level 2 model, why bother with them? Again, this is ofcourse wildly dependent on species and faction traits. But for kingdoms with a high base cost, like avian, if you want to even slightly customize your species with some traits, the cost of higher level classes skyrockets, while you aren't getting much in terms of staying or hitting power in return.
Biests Level 2s are not as strong as other level 2's, but their level 1's are quite strong in comparison to the other domains.
Something to consider if you do not have a plan for a level 2 is to make your level 2 a monster using Monster X. This is something done quite a bit.
We recently spent far too long here a few days back ranking classes among the Domains. Each Domain has has some levels much stronger than others across the board. Class levels were never designed as a power level that you can play, but as something unique for each Domain defining how they organize their factions.
General concepts
For instance Insekts tend to have a solid across the board set of classes greatly expanding the variability of what you find. Insekts are also very flexible as they can quickly between games adjust their classes with Rapid Evolution.
Reptilia their level 1's are among the weakest of all Domains. This is while their level 2's are much better. Their level 3's also tend to be strong. Reptilia have an overall flexibility in design to get access to equipment and natural weapons.
Biests have a very strong level 1's and their level 2s are just not much above those level 1s. There level 3's though compete with some of the strongest of the domains. Biests tend to be one of the biggest in terms of just pure characteristics making them a very aggressive domain.
Fey tend to find their standard level 1 disciples to be on the weak side, but make up for it with their subclasses which can select their levels but may only have a single Sphere or rely upon the Paragon (level 5). Fey have some of the most powerful level 3's. Fey have a lot of quirks and odd abilities making them powerful but often with some glaring weaknesses
Humanoids seem like they are always short on wounds, but make it up with being flexible with equipment and traits. They also can jump into other domains the easiest of all the others. Their level 1s are in the middle of the ranking for the domains, but their level 2's while they can be decent still feel rather fragile compared the other domains. Humanoids have great heroes too, and just much more flexible than the other domains.
You will be excited to hear that there are new classes with each domain in the next ages. Even a level 6 Class in the Reptilia!