OneSignal

OneSignal is a customer-engagement and messaging platform built around push notifications, with web push, in-app messaging, email, and SMS layered on top of the same audience and segmentation engine. It is positioned as the default "free push" service for indie developers and startups, then upsells into a multichannel marketing tool for mid-market teams. Its core differentiator is a genuinely generous free tier (unlimited mobile push sends, unlimited mobile/web subscribers, with only a 10,000-subscriber-per-message cap on web push) paired with broad SDK coverage and a clean REST API. The company operates at very large scale, publicly reporting delivery of more than 10 billion messages per day across over a million apps/companies, and has published a throughput record of 850,000+ push notifications per second.

Where OneSignal wins: time-to-first-notification is short (reviewers commonly cite ~20-minute setup), the SDK matrix is unusually wide (native iOS/Android plus React Native, Flutter, Expo, Unity, Cordova/Ionic, Capacitor, .NET MAUI, Huawei, Amazon, FlutterFlow), and the REST API and docs are well regarded. It is one of the few providers that bundles push, in-app, email, and SMS under one segmentation model, so a small team can run real lifecycle campaigns without stitching tools together. The billing model is usage-aligned for push (you pay per monthly active user / per web subscriber, with unlimited sends), which is attractive for high-send-volume, lower-audience apps.

Where it loses: support is the most consistent complaint, especially on free and lower tiers, where users describe slow, templated, or doc-link responses and report API changes shipping without clear notice. Analytics depth is shallower than dedicated marketing-automation suites (attribution gets murky across concurrent campaigns), and pricing "scales weirdly", the per-MAU model can become expensive as audiences grow even if you are not increasing send volume, pushing larger teams toward custom-negotiated Professional/Enterprise contracts. The 99.9% availability commitment is only contractual on the Enterprise SLA, not the free tier. Net: an excellent default for startups and a strong multichannel option for mid-market, but enterprises with deep analytics or hands-on support needs should scrutinize the upper tiers.