HTML in posts has been discussed many times before and it usually comes down to not being a sensible solution for the problem at hand.
Flarum comes with Markdown and bbcode support by default. A lot of formatting is supported, but sometimes only in bbcode. So in the situation of an import, it's usually possible to convert from another format and retain most features. The problem might be finding a good migration script. Since bbcode has the most features, it might make sense to look for an HTML to bbcode converter.
Tables are not a native Markdown feature and are not in the basic version Flarum ships with. However there's an extension to enable the "pipes table" syntax for Markdown https://discuss.flarum.org/d/17770-friendsofflarum-formatting Make sure the extension is enabled before importing in Flarum, because the post content isn't automatically parsed again unless you edit and save a post.
Our formatting library also has support for pseudo-HTML, but it's not enabled by default. This is similar to how GitHub allows a very limited subset of HTML inside of Markdown. An extension could be made to enable that feature https://s9etextformatter.readthedocs.io/Plugins/HTMLElements/Synopsis/
For a legitimate use case where the post really needs to contain custom HTML, an extension could allow switching post type and changing the rendering engine completely. But it's a very complicated extension to maintain with all the additional protections that must come with allowing user-provided code.
