From my experience (admittedly with independent email services that were around before Gmail was even a gleam in Larry Page's eye), Gmail is only a modest fraction of the problem. Other big players - especially Microsoft - are generally worse.
Flip-side, there seem to be more spammy messages sent from @gmail.com addresses than from any of the other email A-listers.
Agreed. I self-host for my and my friends' personal and project domains, and delivery to gmail works. Granted, nothing is commercial and the volume is so low that rate limiting is not an issue, but if you set things up properly, they'll take your mail, and if you don't, they're pretty good about telling you what's wrong.
On the other hand, it's simply impossible to satisfy Microsoft. We're irrevocably tainted by being in a netblock of a well-known provider, despite having held the same IPv4 address clean for over a decade.
For what it's worth, I managed to get whitelisted by Microsoft a few months ago after... 15 years of undeliverability or so.
I followed the process, and then kept insisting a bit by answering the emails saying they were not going to do anything and I had to check if I was complying with their rules etc. After two emails I had a real person answer me, and a few more emails later (basically insisting I was already enrolled in their various bullshit spam reduction programs and there was zero spam problem with my domain) I got told that I had been whitelisted.
As a user, O365's default spam filtering was just terrible about 2 years ago. I got so many false positives that I had to check my spam folder multiple times per day. I ended up adding very aggressive domain whitelists because I was so tired of it.
FWIW, a client of ours got their office mail server off MS's blacklist a few years back. In less than a week. But that seemed to require their ISP (a mid-sized firm in the Midwest, with awesome customer service) going to bat for them with MS.
I'm guessing some of the issues are just from Google being random about what it considers spam no matter who it comes from. I remember a comment on a somewhat recent thread from someone who had to move their business mail away from Gmail because Gmail would classify mail from one paid account at their organization to another under the same organization as spam.
I was just going to say that for the past year I (my server for the whole family) sent maybe 10 messages to google and I personally received at least 15 spam messages from gmail with SEO offers and other kind of spam.
Flip-side, there seem to be more spammy messages sent from @gmail.com addresses than from any of the other email A-listers.