what would it mean if i said "no lo cumpliste"

would it be similar to "me fallaste"

what about "no me cumpliste"

its a situation where someone is an expert in their field but they trip up on something they should not have

Me fallaste would be translated as you failed me.
No lo cumpliste means you didn't finish it/complete it/carry it out.

The use of no me cumpliste will depend on context. If a friend promises to send you a photograph of her pet dog, but forgets to do so, it is OK to say no me cumpliste, which is understood to mean you didn't keep your promise if the listener knows you're giving a reminder about the photo. In like manner, this can be applied to any situation where certain expectations are not fulfilled.

I agree with all that Rusty has said.