1. Small salary and they're not willing to increase it properly.
2. No bonuses, but they'll assure you it's normal. (Spoiler: no it's not, you'll loose more money on it)
3. They won't give you normal projects or even consider your experience. Their only response is "Based on our experience it'll be perfect match for you!", no, it won't. The only good match you'll get is only based on your luck. Their only priority is to sell you to someone and they won't consider any of your problems even if you have urgent surgery.
4. They won't allow you to travel abroad on your vacation, because you'll have to ask their permission and after that you'll probably will get only 2 weeks or so, and travel is allowed only to your home country.
5. Due to the contract based jobs being unstable in Japan you'll only get 3 month contract extensions, which means you won't be able to have any stability. In rare cases you can get 6 months or 1 year, but that's more like an exception. This is bad due to in most cases you'll have to change the city or even a prefecture.
6. Constant contract change may change your living place. Imagine you're lucky and you've got a good contractor job (which almost impossible in this company), nice apartment (depends on your luck) and after 3 month you'll have to abandon everything and go to hell knows where for a different contract. There you won't have a proper apartment (roaches, no normal commuting, no convenience store or markets nearby) and the new company is treating you like a cheap inexperienced slave, if you like that - this company is perfect for you!
7. 60% SALARY REDUCTION. In most cases searching for a contract takes tons of time for them, which means you're jobless for this period of time. Because you're jobless you'll get 60% salary reduction which is illegal in Japan, but they use some schemes to keep it legal. You'll never know in advance that your contract is going to end, why? Because contractors are slaves in Japan. You'll be lucky if you'll get notified 2 month beforehand, usually it's 2 weeks - 1 month. This company is very slow in searching for a new contract, so in most cases you're doomed to get this pay cut. And YOUR TAXES ARE SAME, which means you get same deductions from a smaller salary, that's how they keep it legal, because on paper your salary is same.
8. JAPANESE LESSONS are a LIE. Why? Because while you're contactless you'll get those "mandatory" lessons, which are useless because you're getting thrown in the middle of some classes which started even before you came and you literally have no context in most cases. Even if you have perfect Japanese with N1, you're still considered to be incompetent and you have to take those "lessons". No career education support, it's a myth. They give you access to some super duper basic tutorials (mostly in japanese) and they'll still complain for no one is using it.
9. You're expendable for them, don't waste your time more than a year in this company and don't believe their lies.
10. Tons of rules in the sake of having rules. Which get annoying after a year or so.
11. Constantly mail spamming, which gets you confused when you search for important emails.
12. No work-life balance. You'll have to reply to their email anytime they'll want. If you need something from them, you'll get nothing, but if they need, good luck to you. They won't stop bothering you.