Is it wrong to make assumptions if you're a scientist

But the answer your question: It's okay to make assumptions, if you know what they are. It's the assumptions that you make without thinking about them that sometimes mess you up. I really think scientists are mostly skeptical enough. You can't go around questioning EVERYTHING, all the time. You have to "assume" that the 1 kilogram standard mass that you brought from the precision mass supply store really is 1 kilogram; You have to "assume" that the supplier did a good job of certifying that kilogram. You have to assume that the 10 volt supply that you just measured an hour ago is still putting out 10 volts, unless you have some specific reason to think otherwise.

Newton's gravitation laws (with Einstein's extensions) are tested every few years by several scientists. There are over 30 alternative theories, including one by Einstein himself. So far, all the alternatives have failed to be supported by actual evidence -- with perhaps the exception of the "MOND" and MOND-like theories. These are still an area of exploration by several scientists.

Also, scientists LOVE to criticize other scientists work. I would bet that over 1/3 of the reports that scientists write never get published, because they are shot down in the review process.