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).
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).