SQL Debugging on the Spot

Shopify wants per-customer revenue and order counts for completed orders, broken out for a finance review. A teammate wrote the query below and it's already been flagged as producing numbers finance doesn't trust.

CREATE TABLE customers (
    customer_id  INT PRIMARY KEY,
    email        TEXT,
    country      TEXT
);

CREATE TABLE orders (
    order_id      INT PRIMARY KEY,
    customer_id   INT REFERENCES customers(customer_id),
    order_status  TEXT,   -- 'completed', 'cancelled', 'refunded'
    total_amount  NUMERIC,
    created_at    TIMESTAMP
);

CREATE TABLE order_items (
    item_id     INT PRIMARY KEY,
    order_id    INT REFERENCES orders(order_id),
    product_id  INT,
    quantity    INT,
    unit_price  NUMERIC
);

SELECT
    c.customer_id,
    c.email,
    COUNT(o.order_id) AS order_count,
    SUM(oi.unit_price) AS total_revenue
FROM customers c
LEFT JOIN orders o ON o.customer_id = c.customer_id
JOIN order_items oi ON oi.order_id = o.order_id
WHERE o.order_status = 'completed'
GROUP BY c.customer_id, c.email;

Type out what's wrong with this SQL code — every way its numbers are wrong or misleading, not just the one that's easiest to spot.