What are the drawbacks to have computers in a domain and users/ressources in a child domain ?

Hi,

we have an Active Directory domain oganized like this :
1. DomA with approx 1500 users; 1000 computers and 120 servers
2. DomB which is a child domain of DomA with 500 users, 100 computers and 20 servers.

DomB is a very old domain and we will migrate in few months every ressources (users, computers & servers) from DomB to DomA

But actually, we want to migrate ONLY computers from domB to domA (when I say migrate, I mean "manually change the domain configuration" from each computer (as we also have many specific tasks to do on each ones).

So my question is :
What are the drawbacks to be in such situation ? (so with users from DomBlogging from a computer member of "DomA" and accessing ressources onDomA and DomB.

I dont really think it's a good situation but I dont have enough arguments to convince my boss ;-)
Many thanks for your help