After playing 2 Beginner campaigns and one vet campaign, i think the balance from beginner to vet is a bit of a jump. But saying that, it is not unplayable.
Sure sometimes you will get unlucky and ground into the dirt, but it makes the fights more interesting.
Playing Vet has definitely made me play smarter, using nets and better skill/weapon/perk assignments for my mercs. I have to admit if there was a ‘Normal’ difficulty level (between beg and vet) it might make a nice balance for people having issues with Vet yet finding beginner too easy.