I'm afraid I've never used it with direct Activesync; my Exchange connection has always been EWS/OWA-based (whatever they call the web endpoint now in O365 hosted Exchange).
Absolutely, it works (and has worked) with EWS for me reliably over the years, I've never actually used ActiveSync on my (personal) devices, as that gives my company control (MDM) over my device to remote wipe it. I've used AquaMail for... 8 years? (back when it was just one dev and $5) across all the Exchange movement my company has done.
It did take them (the new owners) awhile to get the O365 MFA support coded in, which is when I purchased Nine and used it for a year or so. Once AquaMail got their code updated to handle that, it was like a breath of fresh air coming back to the client. (the export/import of your settings - which are plentiful - works great, I've also upgraded phones many times)
