Pingdom (original), Checkly, etc. are for monitoring your websites, from "outside", with a focus on availability (super frequent checks, from different geos, ...).

Datadog, New Relic, Pingdom APM, etc. are for monitoring your production environments, from "inside", deeply integrate with your code, with a focus on availability and troubleshooting (stack traces, distributed tracing, ...).

Assertly is for monitoring of your systems (production, or custom in-house internal, or licensed SaaS) and making sure they behave in a way they're supposed to, with a focus on security, integrity, compliance (more complex, customized checks that run for example daily).