I think the one downside of Haxe (which is also is strength) is that it doesn’t only support compiling to C/C++, but to every major language possible (Java, C#, PHP, Javascript), so the language ends up being too complex. You constantly have to think about how the languages features will map to your target language if you care about performance, which can be pretty unintuitive. And the C++ target (hxcpp) is quite heavyweight and takes a lot of time to compile (at least the last time I checked out). Also C interop doesn’t seem that convenient.
On the other had Nim only transpiles to C, so the language is simpler and easier to reason about performance. (And it probably has faster compile times). It is also explicitly designed to interop with C, so you can easily use all those C libraries in the wild...
But I still have some hope on Haxe, because the main developer is working on Hashlink, which can run Haxe code on a virtual machine but can also transpile it to C for production builds... It seems a more simple and “focused” Haxe target that doesn’t have all the warts of hxcpp...