We currently run on Clientexec. While Blesta is quite feature rich, it is a long way away from having the kind of plugins and compatibility that WHMCS has. Blesta is quite new (2-3yrs) in the making. I'm hoping that couple of years down the line if they keep going at the current pace, things will look much more competitive.
While WHMCS is kind of the industry standard, you must understand one thing. Their support is literally Zero. Unless you are willing to wait for upto 24hours or actually pay $$ for each ticket you raise, the support is non existent. Yes, there is a lot of documentation around all the issues with WHMCS however, you will still have problems unique to your setup which can only be resolved with support and that's the drawback for them I guess. Also, they have moved to a billing model based on number of clients instead of a standard charge.
Also, remember, WHMCS has the same problems windows has which is because its so widely used, the security exploits are so frequent and there has been quite a few incidents of poor security implementation causing huge losses to few hosters. So, consider all the issues holistically before making your decision.