Re: Eppo - they're similar in setup but Growthbook is open source with an optional cloud SaaS offering. Eppo is currently cloud SaaS only AFAIK.
Re: Optimizely - Optimizely is a walled garden, you need to push all of your metrics as events to Optimizely from your apps for any experiment analyses. Eppo and GrowthBook both connect to your data warehouse and support any metrics you can design in query form using your existing warehouse.