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: 
The great advantage of boxing and wrestling is that boxers and wrestlers practice actual fighting, with all other elements of their training - weights, bag work, jumping rope - in support of improving them as fighters. Even when practicing a constrained format a boxer's conditioning, strength and (most importantly) familiarity with hard contact will invariably see them roll over the top of more esoteric fighting methods. Regardless of the style, the single greatest determinant of effectiveness is hard, realistic sparring. If you want to learn how to fight, practice fighting. It's a no brainer
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.