what are the limitations of ATF?

[smilinrobin]

What are the main limitations of ATF? And why would people buy testing tools for testing Servicenow, when its offered out of the box? 

Ive done some research , and their are decent blogs from vendors, but I wanted to know from the community, so asking here. 

[AutomatePro]

Good question, and honestly the answer is "it depends what you're testing and who has to trust the result."

ATF's real limits aren't a feature gap that's going to get patched next release — they're structural, because it's a platform app running inside the platform:

• First reason is gaps in ATF coverage - especially testing the UI/UX.  Certain parts of ServiceNow can't be tested with ATF, especially customised or heavily configured pages.  ATF doesn't do true browser-based UI persona based testing — it can't validate what a user actually experiences in Workspace, portal flows, or certain UI actions the way a real browser-driven test can. If you want to test workflows rather than functions, ATF can't fully do it.
• It can't reach past ServiceNow's boundary. If your processes or workflows involve interfaces/APIs and handoffs to SAP, Salesforce, Workday, etc, ATF stops at the edge of your ServiceNow instance.  A gap ServiceNow admit and say they are never going to fill on ATF.
• Running tests at scale is an issue in ATF.  ATF Test Runner (a chargeable ATF licence) uses headless test execution which often leads to inconsistent test results.  Sometimes the tests pass; sometimes they fail, so you can't rely on the results.  ServiceNow haven't nailed this problem and it leads to distrust in ATF results and abandoning use of the tool and a search for an alternative.
• Test evidence and documentation.  A gap ServiceNow admit and say they are never going to fill on ATF.

Where ATF is genuinely the right and cheapest tool: server-side logic, business rules, script includes i.e. unit style testing rather than testing workflows.  Anything below the UI layer where it has native access most external tools structurally can't get (and honestly shouldn't, from a security boundary point of view).

Why people still buy something else on top: for a few reasons. Their processes/workflows are heavily customised past what out-of-the-box UI testing covers, or they're in a regulated environment where "a tester said it passed" isn't sufficient — they need an evidenced, audit-ready trail of what was tested and what the result was, which isn't really what ATF is built to produce.  They need scalability, reliability and repeatability of testing.  They need to test beyond the boundary of ServiceNow i.e. they need enterprise grade independent test automation, rather than a free tool from the platform vendor.

If none of these are true for your org, ATF plus good discipline could be enough.  If they are true, then something like AutomatePro may be worth exploring (see SN app store AutomatePro - ServiceNow Store).