> One big downside is that is has a margin of error which is very often unacceptable.

You should look at the paper they mention The Case for Learned Index Structures[0]. It covers B-Tree-Index and how to use Hybrid Indexes to bound the errors so that "bad" answers from the model perform no worse than the standard data structure.

For a lighter read this paper was also covered by Adrian Colyer in an issue of the morning paper[1].

[0] https://arxiv.org/abs/1712.01208

[1] https://blog.acolyer.org/2018/01/08/the-case-for-learned-ind...

[edit: formatting]