Limits will be hard caps which we don't be able to exceed. Fortunately, these are project based, so it is unlikely most people, most of the time, will exceed them. (And given how high they are I'd be a lot more worried about the projects!).

Projects will be limited to no more than 700 custom fields and 150 work types. Personally I can't see many groups getting anywhere near those limits as they should be evaluating projects and splitting them up (or removing fields) when they get larger.

That said, many groups I've worked with don't make time to purge older fields (especially true when groups migrate!). This easily results in older fields that are no longer in use being kept around, which will result in bloat. The fix is both easy (just go remove fields) and hard (no one wants to perform this maintenance).

Guardrails are soft-caps on how many work items or projects you can have in a single site. 

You can't exceed 18,000,000 (that's million) work items and 8,500 projects. Remember that is by site - so if a company legitimately needs more than this you could split up your work by site (e.g. "engineering" gets their own vs. everyone else). So you can easily side-step this cap.

Even if you exceed it you'll be safe - however - over these limits Atlassian can't guarantee good performance... so it's more of a "try at your own risk" vs. "you can't do it".

Most groups will never get near these limits so most of us are safe!

That said, for those of us in bigger environments we may come up close to them VERY quickly. This is where it's on us as Admins to keep an eye on how things are going and to take proactive steps to minimize our footprint.