Register.com is the worst domain name registrar I've ever experienced. The service is unreliable, their customer service is outright horrendous and their tech support is incompetent, understaffed or both.
I had the displeasure of trying to resolve a DNS propagation issue with Register.com. Every customer service representative I spoke to had different excuses and details. They constantly misled us about the progress of resolution and kept back the timeframe. No details were ever explained, Customer Service continually provided vague explanations such as "This has been escalated to Senior Admin." When I asked to speak to a manager I was treated with hostility. The DNS propagation was was supposed to be resolved in 24-48 hours. As of this review it is 120 hours. Our business got to a point where we had to transfer our domain name registrar. At that point they stopped work completely.
Regarding the transfer of our domain name registrar, ICANN standards are 5-7 business days. Register.com Customer Service team told me that our domain would be released in 7-10 days, a clear violation of standards.