One of the beauties of Iyengar Yoga is that it can be adapted for specific needs so that most people can practice and benefit from it. Under the right circumstances, and with the right skilled teacher, I believe that anyone who is interested can safely practice this system of yoga.
That's not to say that any particular Iyengar yoga class will be able to accommodate anyone who comes to that class. Certain classes will focus more on general beginning level skills, while others are geared toward experienced practitioners of Iyengar Yoga. Some classes may be structured for yoga students who need a more gentle and supported approach to the practice of yoga poses, while other classes may be very vigorous and difficult.
The classes aren't "one-size-fits-all" classes. They can't be. Beginners need to learn basic elements of this practice before moving on to higher level classes, and more developed students need the opportunity to go deeper into their practice, under the watchful eye of a good teacher. Students with physical difficulties, disabilities, or chronic health conditions do better with a more careful approach to the practice of yoga.
A teacher can't do justice to all of her or his students if the range of abilities and needs within a class is too wide. Of course it wouldn't be fair to a new beginner with mobility issues to be expected to keep up in a class of a vigorous sequence of jumpings, arm balances, and long inversions. But it also wouldn't be fair to more experienced students to have a new student who's not experienced in a particular style of yoga, such as Iyengar Yoga, expect to join them in an advanced class while not knowing the basic elements of that system.
If you're new to yoga and want to take a class, check the requirements for the different classes. If you have yoga experience, but want to try a different style of yoga, also check the class requirements. You may be very skilled, strong, and flexible in a certain yoga practice, but you may not know enough to do well in a higher level class of another yoga system (and you might not know enough to realize you're not doing well...). Yes, a good teacher can handle a wide range of abilities, but you'd be doing the rest of the class a disservice if the teacher has to focus most of her attention on helping you because you don't have the required background for that class. You might even look a little foolish if you're in over your head and don't know it. Ask the teacher if you're not sure if a certain class is appropriate for you and your experience level. Don't feel insulted if the teacher wants you to take a lower level class at first. Ultimately you will be more grounded in the basics of that yoga system, rather than superficially trying to practice at a more advanced level.
Different styles of yoga have different expectations in classes. Do your homework to get a good sense of these expectations, especially if you're thinking of trying a style that is new to you. Be humble and don't expect to jump into the highest level class, even if you are very skilled in another method. You will ultimately learn more this way.
Think of it this way -- a fiddle player will have very different skills from a symphony violinist. They both may be excellent in their area of practice, but they wouldn't expect to be able to switch venues without going back to a more basic practice first.