Question: I've started Wing Chun few days before. Everything seems to be nice, but I've seen some videos on internet, they say that Wing Chun can't handle Boxers or Wrestlers.

What are the comparisons between Wing Chun and boxing/wrestling that would lead someone to make this claim? Are there weaknesses that can be exploited?

Answer: 
I don't think that it makes too much sense to compare styles instead of individual athletes, but I will try to answer your question anyway.

There is no one answer to this as it is very hard to say how important the advantages and disadvantages of both styles are. Therefore I will give you some insight to the advantages/disadvantages and you can think about it yourself.

Wing Chun has good punches, which may be the fastest with the least amount of telegraphing, but they have less range and have a less power compared to the punch of a boxer (they are still powerful and hurt).

But the most important thing is the range. Wing Chun and Boxers have different ranges in which they work and feel comfortable. Also because of the range, they have different tactics to be in that range.

A boxer normally stays back, out of range, moving in to strike then moving back again.

A wing chun practioner will be much closer to their opponent and tries to stay close (this is one reason why "sticky hands" are important).

I assume that the fighter who can stay at his preferred range will win the fight. Wing chun focuses on sticking to the opponent, maintaining physical contact in some way. The sense of feeling is 6 times faster than the sense of seeing, which obviously gives a strong advantage to the wing chun fighter in their preferred range.

Some boxing schools start training in the wing chun range. If this is the case, the boxer has an advantage, because all boxers I know train a lot to get into their range. 

An additional point I can imagine that a boxer could lose because he underestimates his opponent in general. On the other hand I see a good chance, that the wing chun practitioner gets knocked out by a hook, because I have seen a lot of wing chun people underestimating every round technique.

Wrestling and wing chun are even more difficult to compare, but I think it's about the same as with a boxer versus a wing chun guy.
So there are some fundamental assumptions here that one must look at with "intent or goal".

**Goal:
To beat your opponent senseless?**

* If this is your goal then you will probably want a style that focuses aggression and subjugation of your opponent over anything else.

**Goal:
To protect oneself?**

* If this is the goal then any discipline which focuses on body mechanics as well as properly analyzing a situation and working to diffuse it works well.

**Goal:
To develop oneself through discipline and mastery of ones body?**

* If this is the goal then the ability to "beat someone" isn't really as important as the type of person you become through the exercise and mastery of the art. Boxing, Wrestling, Kung Fu, Tai Chi, Karate, etc... all have the ability to develop ones physical and mental techniques while disciplining and harnessing ones own inner self for personal growth.

P.S. Unless you are practicing just for a MMA full contact event, then any style has use, purpose, and value. The only caveat to add is if you are grappling it's hard to hit and if you are at a distance it's hard to grapple, so there is a body mechanics balance needed to be well rounded.

P.P.S. As far as ability to beat another opponent, this is largely circumspect on the individual's abilities and if one is better skilled and physically capable then they will likely win regardless of the style in question.