Guarbards are stand-alone types. Which means, if your bards get incapitated. You can be the back-up but it's quite difficult balancing between heals and maintaining aggro. So far, this kind of situation only happened to me once.
As for Guardagers, you gain access to combo skills that has long CD, and also to aoes, more dps but if you don't have the right build to back up you being unable to reduce dmg taken. You might reconsider going Ravager as secondary. But I'm sure with a competent bard, Garudagers can do fine.
"Guard/bard will have a hard time keeping aggros while guard/rav will have constant aggros.."
As a Guarbard, I don't have trouble with maintaining aggro, since I use Merrilee and spam her combo if ever aggro changes. But it only seems to happen when someone decides to want to reset malice using Gigas and Sigrun which can be annoying to juggle aggroing. Also, if you have a good skill cycle, you can maintain aggro either way.
Mirror Shield>Roar (reset aggro)>shield bash>Slash>thunder cut...Something of that sequence.