Not experienced with HOHO in Cartegena. However, used them in multiple cities in Europe and US and have some general advice. Some cities seem better organized or arranged for HOHOs and some don't. There are multiple reasons, but some include traffic congestion, poor scheduling of drivers and buses, too few buses, how many tourists and ships in port, etc.
Checking Trip Advisor (sister company of CC) there might be a problem with scheduled stops in Cartegena. In other words, you might be waiting a long time for a bus when you could be seeing the sights. This is critical for us as cruisers as our time is limited in port. We opted not to use the HOHO in Cartegena and went with a tour instead.
I hope this helps shed some light on the HOHO issue.