Can the hotel booking API give me guest reviews and ratings so my customer can judge a hotel before booking?

Sometimes yes, but this is one of the areas where suppliers differ the most, so you cannot assume every hotel in your portal will come with reviews.

Where review content is included you usually get an overall guest score, the number of reviews it is based on, and often a breakdown by things travellers care about such as cleanliness, location, the staff and value. Some suppliers also send the written reviews themselves, with the date and the type of traveller. Keep in mind that this guest score is a different thing from the star rating, which is the official classification of the property and tends to come through on almost every feed.

The limits are mostly commercial rather than technical. Review content is often licensed from a separate provider, so it may only be part of certain plans, and the supplier may set rules about how you display it, such as crediting where it came from or not mixing scores from different sources into one number of your own. Reviews are also not always available in every language, and a small or newly listed hotel may simply have none yet, which is not a fault in the feed.

If reviews are important to the way your customers choose, ask which of your suppliers include review content and what the display rules are, then look at a mix of large chain hotels and smaller properties in your own portal to see how often a score actually appears.