Re. the ultimate end results...yes, you stand a chance at upgrading a clone to perform 1) better than other stock airsoft guns, 2) better than it started out as, 3) as good as a upgraded "good" airsoft gun.
The crux of why so many people (myself included) say "don't start with a low end clone" is that the deck is usually stacked against you to do so and end up with result #3. If you expectations are #1 or #2...then have at it, you're likely to be successful to some degree.
If you're scratch building something that cannot be made from a base gun (i.e. you want that triangular barrel with that octagon receiver and that comp set trigger on a uber rare stock and you're going to pick that PDI hopup/barrel/piston/cylinder set anyways)....the scratch build it or start with whatever base gives you the best of the remaining parts you're going to need (pretty much just the bipod and sling studs at that point...LOL).
When dealing with clones and copies of clones you tend to deal with parts that are off center, thinner, oddly spec'd....in addition to the parts being made of cheaper materials (in general). There's no correcting a skewed/oddly threaded receiver and still use the other commercially available outer barrels. Weird fitment issues such as the receiver being too thick to position a Zero trigger properly can/may be fixed by filing/grinding areas. Other issues such as an upgrade cylinder not fitting in the receiver spacers can be a pain. Alignment and dimension issues where the stock/upgrade cylinder tip may not be positioned properly with the stock/upgrade hopup unit to create a good seal can drive guys nuts.....and due to the usually very expensive BA upgrade prices it's usually a bitter pill to swallow.
Although the same thing goes with AEG clones...their overall quality/consistency has improved.
The potential headaches and struggles of upgrading a clone BA is usually why people recommend not doing it.