The verdict
For most small B2B teams, I'd pick Manychat if Instagram, WhatsApp or Messenger is already a real source of enquiries. It has a free plan, the builder is approachable and the contacts flow into HubSpot or an email tool. I'd pick Customers.ai only if you run an e-commerce brand on Klaviyo with plenty of traffic and a stalled list.
The risks differ. Manychat depends on Meta's rules, so keep your email list as a second route to your audience. Customers.ai raises consent questions, especially in Europe, where you need a clear legal basis before you contact people who never signed up.