I write both Erlang and Elixir. One negative thing I'll say about Elixir is that it sometimes feels like a Ruby-shaped DSL obfuscating the Erlang I would otherwise be writing more clearly.
I don't like optional syntax. Like dropping parens for argument lists, etc. Elixir adapted this quality of having varieties of optional syntax from Ruby-land, and I think it makes things harder to read and writing is sometimes ambiguous. So for my part I just avoid it and write out the explicit form every time.
I also think the weird variable rebinding doesn't really solve an interesting problem, but also ends up sometimes making things more confusing when reading code in projects written by people who leverage it all the time. I happen to like explicitly naming reused constructs with different names each time they're bound. 1) it makes is explicit which version of a thing you're working with (e.g. Time1, Time2, etc.), and 2) it makes writing the code a lot more similar to how one might sketch up a more formal construction (e.g. T, T', etc.). So here again I just write out the explicit form like I would have in Erlang. To me the very existence of the "pin operator" is kind of an indicator that rebinding was a weird design choice to solve what seems like a non-problem.
Lastly there's a fair bit of boilerplate in Erlang when using the OTP patterns, and in Elixir there are often metaprogramming constructs that attempt to provide some sugar or some shortcuts to mitigate that, but I find this is really hard for newcomers when trying to understand how something like say Supervisors work, because in Erlang everything that's going on is explicit and well documented, but in Elixir the connections and relationships between concepts, callbacks, etc. isn't quite as clearly spelled out and the docs don't really help much (in fact they make it more confusing because at the same time they introduce Supervisors they also introduce GenServers, and give the false impression those concepts are tightly coupled). I often point learners at the Erlang docs and give some instruction around OTP concepts in Erlang so that they have an easier time mapping them in Elixir.
That said there are a ton of things I really like about Elixir. Starting with Mix. Oh my god how I love Mix. Having the language come bundled with an easy to use build, dependency, and release management tool is unbelievably nice. I also really like the fact that a lot of effort went into organizing the standard library so it's coherent. For the most part things are pretty discoverable. Functions that take state or context to modify pretty much always have it passed in in the same location in the argument list. Whereas in Erlang it's different depending on which module you're in, and sometimes not even consistent inside that same module. Also it's nearly impossible to know where to go look for a particular piece of functionality in Erlang due to the way the organization of the standard library accreted over many many years.
I've really enjoyed adding Elixir to my stable of tools, and there are a couple projects at work will definitely be made significantly richer by the fact that they're built in Elixir.
Oh, and I also really miss that variables in Erlang all begin with a Capital letter. If Erlang had Elixir's atom syntax, or Elixir had Erlangs variable syntax, I'd be in code heaven.
Then functions, variables, and atoms would all be trivially easy to identify at a glance.
I don't like optional syntax. Like dropping parens for argument lists, etc. Elixir adapted this quality of having varieties of optional syntax from Ruby-land, and I think it makes things harder to read and writing is sometimes ambiguous. So for my part I just avoid it and write out the explicit form every time.
I also think the weird variable rebinding doesn't really solve an interesting problem, but also ends up sometimes making things more confusing when reading code in projects written by people who leverage it all the time. I happen to like explicitly naming reused constructs with different names each time they're bound. 1) it makes is explicit which version of a thing you're working with (e.g. Time1, Time2, etc.), and 2) it makes writing the code a lot more similar to how one might sketch up a more formal construction (e.g. T, T', etc.). So here again I just write out the explicit form like I would have in Erlang. To me the very existence of the "pin operator" is kind of an indicator that rebinding was a weird design choice to solve what seems like a non-problem.
Lastly there's a fair bit of boilerplate in Erlang when using the OTP patterns, and in Elixir there are often metaprogramming constructs that attempt to provide some sugar or some shortcuts to mitigate that, but I find this is really hard for newcomers when trying to understand how something like say Supervisors work, because in Erlang everything that's going on is explicit and well documented, but in Elixir the connections and relationships between concepts, callbacks, etc. isn't quite as clearly spelled out and the docs don't really help much (in fact they make it more confusing because at the same time they introduce Supervisors they also introduce GenServers, and give the false impression those concepts are tightly coupled). I often point learners at the Erlang docs and give some instruction around OTP concepts in Erlang so that they have an easier time mapping them in Elixir.
That said there are a ton of things I really like about Elixir. Starting with Mix. Oh my god how I love Mix. Having the language come bundled with an easy to use build, dependency, and release management tool is unbelievably nice. I also really like the fact that a lot of effort went into organizing the standard library so it's coherent. For the most part things are pretty discoverable. Functions that take state or context to modify pretty much always have it passed in in the same location in the argument list. Whereas in Erlang it's different depending on which module you're in, and sometimes not even consistent inside that same module. Also it's nearly impossible to know where to go look for a particular piece of functionality in Erlang due to the way the organization of the standard library accreted over many many years.
I've really enjoyed adding Elixir to my stable of tools, and there are a couple projects at work will definitely be made significantly richer by the fact that they're built in Elixir.