St. Mother Teresa’s experience is not a typical one for the vast majority of even devout Catholics. If we pursue the interior life seriously we all will likely have a few dark nights- it’s just a part of the spiritual purgation that has to occur as we grow nearer to God. Think of it like finishing a piece of wood- the more attention we pay to the sanding process (purgation) the better the final project will be once the finish is applied. Or you can liken it to strength training, which involves repeatedly breaking down muscle to encourage growth; not always a pleasant process but the ends are far better than the soreness and time spent training.
Regarding her detractors, many atheists who hate on her do so thanks to Christopher Hitchens and his work. If you aren’t familiar with him he was a bastion of atheism and highly critical of the Church- read some of his work and you’ll see how misguided and probably miserable he was. In my estimation Hitchens may have had good intentions with calling out Mother Teresa, however he do so because he was operating under the wrong assumptions. He made little effort to understand her work and mission in India and so he inaccurately judged it. It wouls be like if you started to remodel your bathroom and a friend came over and saw nothing but bare studs and unfinished plumbing and accused you of ruining your house, without ever taking into account that, when the project is done, you’ll have a bathroom (a better one) once again.