One of the best pieces of programming advice I got at Google was consider IDs carefully. This is not a solved problem, and there isn't a one-sized-fits-all solution. Rather, there are a bunch of guide rules, and then you need to understand your problem domain very well to choose good identifiers.

An ID should be:

1.) Actually an identifier, i.e. it needs to identify objects uniquely and must be immutable throughout the lifetime of the object.

2.) Assigned at object-creation time; you can't have an object without an ID (in persistent storage), lest you'll have no way to refer to that object later. This presents particular problems for a lot of workflows where you want to introduce the concept of a user early, before soliciting personal data. Think of storing browsing histories for guest users, or persisting shopping carts before checkout.

3.) Integral to the storage system. IDs will usually be the ways that you lookup and join objects; the performance characteristics of your database can influence the type of data you choose for an ID.

4.) Because of #2 & #3, oftentimes a significant concurrency bottleneck for object creation. This is the downfall of many auto-increment integer schemes.

5.) The foreign key for other objects. This has space implications that you often have to trade-off against latency implications. If you store useful information within the ID, you can use that info without needing to make a separate query or join against the DB. However, you need to carefully consider whether that'll run afoul with #1, and the more information you put in the ID, the bigger the size bloat for other objects that reference that ID.

6.) Oftentimes a security & PII risk. Because IDs are the primary means for lookup and are guaranteed to be unique, it's awfully tempting to put them in your public APIs (like HTTP URLs). But then anyone who has a URL has all the information included in the ID. If you use sequential IDs, they also have the ability to scan your entire database; this was the downfall of Parler.

Integer keys do really well for #1, #2, #3, and #5, but are very problematic for #4 and #6. But then, there are fairly easy workarounds for those: #4 is often solved by hashing a unique natural key of the object (also solving #6), while #6 can be solved by never exposing internal IDs to the outside world and instead using a lookup table on some friendly URL scheme, which is better for UX anyway (at the cost of #3). GUIDs do well for #1, #2, #4, and #6, but perform worse for #3 and #5.

Natural keys (where you make the identifier some combination of the actual data) are also frequently underrated. The most obvious use for these are relation tables, where each row just indicates that two entities have the relationship that the table describes; you wouldn't normally put an auto-increment ID on that. But think also of something like a search refinement: the most natural key for that is [query, language] => [list of suggestions], and that uniquely identifies each set of refinements, and it has other useful properties where the rest of your search engine doesn't need to know that refinements exist (it already has the query), and you don't need a lookup call from query to some search_refinement_id or vice versa, and you can easily enumerate the set of languages that a given query has data for, and you can follow a chain of refinements without any intermediate lookups. If you were to suggest either integers or UUIDs for this problem I'd say that you're overengineering. And if the problem domain changes such that the key-set changes (for example, you want to include past search context, or you want to personalize refinements to each user) then I would recommend you start a different system from scratch and eventually replace or merge in the current one, because those problems have sufficiently different requirements that your whole data pipelines are going to be different. (In particular, personalized search requires PII handling, a lot of care in logging, encryption of the data when at rest, knowledge of a lot more entities in the system, etc.)