Are you saying with your host there is no chance a high bandwidth neighbour can move in and mess things up?....Or has it been that you have just been "lucky" so far?....
any number of things can mess your services up, I'd be more concerned about local resources than bandwidth. the vast majority of your shared hosting problems will have nothing to do with bandwidth, rather I/O, RAM, Disk Space, etc. you have the same risk with any platform, be it shared, VPS, dedicated or otherwise... any host with a decent set of round-robin sonicwalls can be taken down by a DDOS which will affect all services.
with that risk existing in nearly every offering, it's more important to look at the state of overselling, the load averages on the box and available resources.
for example, the server I'm on has plenty of memory and 4 xeon processors with enough hard drive space to handle everyone's storage... as well as 4x 100mbit carriers. it's all about mitigating risk and handling what you can, preparing as best as possible for what you can't control. finding a host that actually cares and doesn't see you as a number is the hardest part- if they don't care, a server that could handle 300 accounts of a certain size will house over 1000 and hurt the performance of all on the server, just to increase profits.
roughly, that's why I've finally settled on a hosting home.