I can only guess why you might be feeling uneasy. I've no experience. It seems a sort of intangible unease on your part that you can't put your finger on.
I'm wondering if the reason you might feel like this is because, to you, it might be a bit not in character for your ex, and you might feel it has been a case of some pressure being applied to him. The reason for formally adopting may not feel, to you, very organic - in view of what you may know and think about timescales of his relationship, or other aspects of the situation.
It's also sometimes the case that exes, or siblings tell close ones, of a decision they are making, when they don't really need to, and sort of confide and justify themselves, as a way of subconsciously seeking guidance, as they are not quite sure of what they are doing and of good reasons for doing it.
I suppose it depends on how long he has known his partner and whether you feel the situation seems forced. It's a bit pointed perhaps, in your mind, to go through a legal process, when it's not obvious why it would be essential. Especially if the relationship with his new partner is fairly recent, you might worry I guess. You might worry at the back of your mind that the only reason to adopt would be inheritance issues. Only you know if you might feel this. If you acknowledge that your ex might have biological children in future which would be no different, and if you think your child wouldn't mind emotionally, then I think that your misgivings are based on a feeling that adoption is about inheritance.