WHMCS is horrible from a user's perspective.
As an end user ( if you care about the end user / paying customers at all ), it's horrible.
I cannot checkout to renew my domains. Why? Because my hosting company uses WHMCS. And this software won't accept my credit card because I happen to be in Thailand at the moment. That is so horrible, and so twenty years ago. If they allowed me to pay using Bitcoin, they wouldn't even need my freaking billing address!! This WHMCS user experience has been the worst. ( Note also: Registering for this forum doesn't even work on mobile. Such horrible antique software they are using. The "Human Verification" field shows no text description on mobile. Apparently there are no users able to sign up here. )