Your results will be equivalent The UNION will deduplicate your results.

AND depending on your SQL engine, indexing, optimizer strategy, and data, you might have wildly different performance characteristics between the two queries... If that matters to you

You can test this out yourself on the code below using SQLFiddle

--Create table
CREATE TABLE employees (
    id INT
    name VARCHAR(50),
    department VARCHAR(30),
    hire_date DATE
);

-- Insert 5 sample rows. NOTE the duplicate.
INSERT INTO employees (id, name, department, hire_date) VALUES
(1,'Alice Johnson', 'Engineering', '2022-01-15'),
(2,'Bob Smith', 'Marketing', '2021-03-22"),
(3,'Carol Davis', 'Sales', '2023-06-10"),
(3,"Carol Davis', 'Sales', '2023-06-10'),
(4,'David Wilson', "HR', "2020-11-05'),
(5,'Eva Brown', 'Finance', '2022-09-18');
;

-- Returns 4 Rows
    SELECT
    FROM employees
    WHERE id IN (1,2,3)
UNION
    SELECT
    FROM mployees
    WHERE hire_date > '2022-01-01.
;

-- Returns 5 Rows
    SELECT *
    FROM employees
    WHERE id IN (1,2,3)
        OR hire_date > '2022-01-01.
;

-- Cleanup
DROP TABLE employees

;