I don't disagree with anything you posted above. I should clarify that while I think a killer web interface would be a real success story for SmarterTools that isn't what my signature was about. I have been a proponent of ST looking into MAPI for many years. We've offered SM alongside Exchange servers for well over a decade and, before it, iMAIL. We didn't get MAPI in 15, I had hopes we'd see it in 16 but, as I understand it, implementation was a daunting task.
Outlook support is essential for us. I understand this is true for many others who post here. In fact, I'd go as far as to say that if Outlook continues to be a factor in the business grade email market and ST don't achieve something closer to par with Exchange it will turn out to have been a very bad expenditure of capital and other resources for ST. That opens some real cans of worms plural. Will Outlook even be available outside 365 in the future and, if only in 365, will it work with 3rd party Exchange-like servers. There are certainly lots of challenges now that seem, coincidentally or by design, to be somewhat resolved by using a retail or volume license version rather than the 365 version. If 365, and if it is still necessary to implement some of the registry patches discussed here and elsewhere, that will be problematic.
I have also seen the problems you list when using Outlook as well as a number of others and have spent many, many hours tracking them down, at times ruling SM out completely as the problem but not so at other times. In the interest of trying to keep this a bit shorter, I'd be happy to share some of the things we've discovered with you out of band. One tip I could share with you though if you haven't tried it (and without exonerating SM as the root cause in some way because I simply don't know) I cannot think of a time recently when I have seen the email still in place on the server but not displayed in Outlook that I haven't found it stored in Outlook but not visible. This is especially so if the Outlook folder structure deviates from Microsoft's recommendations which, among other things, seem to be that the Inbox should be kept small and that more complex folder structures be created peer to it rather than within (below) it. In this case, if you haven't tried it, even just the included Microsoft Inbox repair tool may be your friend. I don't often run it without finding at least some corruption in the file and once that has been resolved and the contents re-indexed things often appear on Outlook that were not visible before the repair.
I am not knocking your observation here just sharing something I have seen frequently enough that it may be helpful. On another occasion recently we could see that email was being successfully delivered into the Inbox but subsequently disappearing even on the server. In that case, and we didn't sort out why it happens, we were able to conclusively prove that it was being deleted by Outlook as a hard delete (no copy in the deleted items folder). Neither a repair or reload of Outlook resolved the problem; however, another one of the many updates that seem to being pushed did.
My great hope is that if we can all keep this constructive, ST will remain committed to evolving SM to a system that is truly on par with Exchange functionality. I am not sure how much longer our client base will indulge the wait so, like you, I may reluctantly need to abandon it. The new licensing Microsoft came up with for SM MAPI makes it difficult to tread water with a smaller client base to try to build it back up again once complete, especially because the Apple guys have been orphaned in such a scenario (we don't have many but few large accounts are without at least one or two).
I think other posts here asking for a frank comparison of the significant features and pitfalls (like losing attributes as part of migration, yikes!) will be critical to having ST installed where it can succeed rather than being shot-gunned in as an alternative before it is ready and being cast back out as a result of the small number of critical users affecting the purchasing decision in an organization loosing functionality they depended on. Some of the rest of it is bugs like all commercial software has and I think ST have shown a real commitment to resolving them. Whether we could catch them before a release was pushed and what processes would be required to do that are a different discussion and I am not as critical as some in that regard although clearly there has been room for improvement.
To reign back in specifically, I agree, even if it were only categories that were not functionally identical to Exchange, I think it would be a big issue for at least some users in most organizations and all it takes is one with significant influence over the purchasing decision to cast what is an excellent system in many other ways out the door. You only get one chance at a first impression and for SM there won't be a second. First priority - bugs of course. Second priority - prioritization of missing features and a road map.