bad flashes. bad wipes. bootloops. failure to make bootable backups.
the biggest issue i see in being active here,the eris,and original droid forum,is that when something goes wrong that you cannot boot into a useable rom,you cannot use rom manager.
bootloops keep you out of a useable rom. ive seen folks boot into semi-functional roms with no rom manager,no market,and no way to side load apps.
the #1 fault rom manager has, is that will not always be there. remember its just an app,so youll need to be in a working rom,with working wifi or data to even consider using it.
recovery is reliable. it will always be there for you. and it is how you will fix boot loops and non-functional roms.
i agree with the original poster 110 %. i came from the eris where rom manager is not really an option,so i got uised to recovery from day 1. i cannot understant for the life if me why folks like to use rom manager.
my advice it anyone who likes to install roms: get 100% comfortable and used to backing up,wiping,and flashing in recovery. at this point if you really feel you need to use rom manager(for whatevr reason

) go ahead. at least youll know how to go to recovery and reflash the rom,or restore a backup to fix it when it goofs up somethin,and we wont see frantic "help im in a bootloop and i dont know what to do!" posts on the forums.
IMO rom manager is unreliable. ill admit it...im a flashaholic. ive prolly installed a couple hundred roms on my 4 phones,and ive never had a situation i couldnt easily recover. my 2 cents