The real question here is: what are your goals? Beasts are okay. They're fun. They're not the toughest or the most damaging, but they're mostly functional.
I feel like you need to make a particular commitment early on with them, though: are you here to pump damage or survival? I think Beasts are a lot more fun when played offensively, with a secondary like Kin or Storm, because they have certain strengths as damage dealers (they run fast, decent single target dps) whereas, defensively, they don't stack up to primaries like Necro, Demons or Robots.
So if you're trying to build a wicked solo machine pushing max difficulty, I wouldn't go Beast. You could do it, but you'll be locked into a defensive secondary and you'll be working more against the nature of your critters rather than with them. Same could be said for Ninjas, maybe even moreso in their case. You can build them tough, but I think it's way more fun with both of these primaries to just stomp the gas and redline it.
Sure, your dudes'll be squishy and it might get tedious resummoning if you hate that, but you'll be able to kill stuff in a much more timely manner and you'll have something to offer fast-moving teams who don't need any defensive support.