It's been getting much better over the years, I remember Pivot Tables making it in being the hot shit in the online version a couple years back, but it's still not to parity for advanced use. The biggest pain point probably being limited data sources though I think there is still a file size limit too (30 MB last I remember, better than at first!). New features still get cooked in the desktop previews first as well e.g. even if your account is set to insider preview lamdba won't work in Excel Online at the moment.