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: 
As always, you can't compare style A with style B and say either one is better. It's not because you're a boxer, you immediately win agains a wing chun practitioner. It all depends on how good both fighters are, how much experience they got fighting opponents of different styles, the competition's rules (eg. if you pit a judoka vs. a taekwondo, and rules state no throws or groundwork, chances are the taekwondo will win. If the rules state throws and groundwork are allowed, but kicks to the head aren't, the tables will turn), and even as much as how good/bad a day someone's been having and even sheer luck (Remember, one lucky hit can be all it takes)

Of course, each style has its strengths and weaknesses. Once you understand your system well enough (and your own body, for that matter), you can start playing with the strengths and the weaknesses. You can get creative with them. That's an important part of the experience part mentioned above.

But yeah, in short: You can't just compare style A with style B. A kid who did 3 weeks boxing classes won't win against Ip Man. A kid who did Wing Chun for 3 weeks won't win against Mohammed Ali.
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.