The first sentence sounds a bit odd to me, probably because failure and victory are words you tend not to encounter together, with one constrasting another. Not knowing the context makes it all the more difficult to understand what the message is you are trying to convey. You are not taking about a person here, obviously, because you used the word „this.“ THIS is more of a failure, etc. The word „failure“ is frequently matched with „success“ in English. „ The operation on the patient was a failure, unfortunately not a success.“ So English speakers are used to hearing failure and success contrasted with one another. Hence, „This is more of a failure than a success“ sounds more natural and expected to an English speaker`s ear. It`s the word pairing that makes it work. It´s like this in other languages too (German comes to mind) where certain word pairings are common, but not others. Failure contrasts with success (as opposites) but victory isn’t seen with failure, or at least it`s not paired with it to my knowledge.
I think the second sentence is fine. The speaker is clearly NOT a doctor. No doctor would admit to „not being much of a doctor.“ The whole idea is simply not credible. You are either a doctor or you are not. But, some people do have some doctor-like skills (say a nurse or well-trained medical assistant). So, a nurse or medical assistant might say, „I`m not much of a doctor, but I can help you with those wounds.“ Even a layperson could say this. If a real doctor said he or she wasn’t much of a doctor, that`s a person I think I wouldn’t walk, but one I`d run away from.