Are there any problems with the fact that email wasn't created to be instant messaging? Are the messages sometimes delayed?

Well, for a starter you will be limited to what email servers provide, which doesn't include video conferences, which most instant messaging solutions include these days

I have not noticed that problem, the messages are delayed by a few seconds, but not noticeably. Only using chatmail / own postfix though, YMMV.

Not in my experience. It feels like chatting on Signal or any other instant messenger, except there are no typing indicators, especially if you use DeltaChat's chatmail server (the default if you just say “let me in”).

I think any competent email provider will throttle or block you the first time you send a burst of messages.

Wait, is that true over regular email too? Say I have a back and forth one liner emails every few minutes? It's competent for the provider to throttle or block these?

They do. All email providers have limits either specified or unspecified. It's usually a few hundred per hour and each recipient uses one quota.

This is why they provide a choice to use their own servers

Then why implement email as a backend if email cannot actually be used as a backend?

That's the tricky part of having email as backbone technology. It's an old and varied ecosystem.

I host my own email server and I am pretty sure chats will be instant with it as well as with their servers.

However, some email providers implement various throttling mechanisms.

If you are OK waiting for message to arrive or be dropped silently, you may continue to use Google and Microsoft email.

I haven't run into that problem, but yes, it's possible.

One problem I did run into was “allowed number of outgoing emails”. If you use groups in DeltaChat, even a small grouop of say 10 members will incur a lot of outgoing messages. The provider I originally used has a limit of 200 emails per day, so that was a showstopper.

If you use DeltaChat's chatmail server (which will happen per default if you don't provide an email account of your own), this will not be a problem.