Ah this is good to know. I current run on-prem Exchange, with a view to moving to Office365, my mailbox is a super-set of all the mail I've ever had and goes back 20+ years.

My email arching ve going back more than 20 years is stored in a single .PST file, and I search it using Outlook. Never had a problem. Every 3 months I copy everything older than 2 months from Exchange Online into that .PST.

Outlook is totally different, and I agree it works perfectly.

I'm talking about OWA Search in O365. I use mostly VDI these days, so PSTs are out. It's a frustrating issue to me because OWA search is better in many ways for more recent stuff.

Also note this is an anecdotal interpretation based on my experience.

Just in case you are not aware, PST files can go horribly wrong when they get to 2GB in size.