Smaller VPS providers like Linode or DO oversubscribe like crazy. Last time I used Linode, they would email us telling us we're using too much CPU or memory, and we'd need to move to a larger tier VM.
I think you misunderstood those emails. They are just there to help you if you didn't realize some process was stuck or something, they specifically say "This is not meant as a warning or a representation that you are misusing your resources." and you can also change the value that triggers those emails or disable them completely.
