I've personally gotten really consistent at up b cancels. I can get 4 to 5 in a row without messing up and that's a raw up b cancel, where you cancel it without hitting someone. If you hit someone with it you get an extra frame or two to cancel. It kind of feels like westballz spamming double shines XD.
Honestly, it's useful but it's not a priority... only practice it when you start getting good at everything else...
Basically... UPB cancel is useful at times... but rarely... and it's risky... if you miss it get ready for a punish. The only match up that you can potentially maybe regularly use it is against fox and eve less captain falcon...
You don't NEED up b cancel to get good with Doc... Shroomed can do it but I have never seen a match where he used it. I've personally have only ever used it in friendlies a couple times and I only once I got a follow up on it.
If you can't do it or at least not consistently... don't worry about it and besides... there's cooler things than an up b cancel like SHBAWD and pill mix ups.