The right lawyer will have time to give your case the attention it deserves. Being organized and timely matters. Lawyers with a heavy caseload may not have the time to represent you properly. For example, you never want to run the risk of missing out on compensation because your attorney was busy and failed to meet crucial filing deadlines.
That being said, having a heavy caseload is not an automatic disqualifier. If you have concerns about whether an attorney you like will be overloaded, ask how much time they expect to dedicate to your case.