My biggest issue with Gmail (as someone who interacts with users of Gmail) - is that it generally breaks quoting and hides this from Gmail users with its magic conversation view.
Outlook(web) does atrocious top-quoting rendering it useless for mailing lists.
In general the web interfaces doesn't work well with hundreds/thousands of mails IMNHO.
They also tend to needlessly lean on html formatting rather than plain text - with things like "my replies in blue" - rather than just proper quoting.
Ed: in general I've just given up the idea that the average user has any hope of interacting well with email lists - which means such lists aren't useful for general discussion (but can still work for specialist groups like Linux kernel etc).
The conclusion being that one will need "something else" for a general audience.
Zulip might be a reasonable compromise.