Not OK, if you are saying this pressure is a negative thing (bad, unfavorable, detrimental or burdensome), and you don't make this clear in the context. "Great" can mean wonderful, as in "The movie was great." Without context, your sentence could be misinterpreted, although you are not technically incorrect, since "great" can mean "a large amount." You could use a negative word in conjunction with "great" to clarify your meaning. For example: The pressure of modern life is overwhelming. The pressures of modern life are more than many can bear. The pressures of modern life are intense. The great pressures of modern life are unbearable. There are great pressures that must be borne in modern life. There is a great amount of pressure in modern life. Modern life puts people under lots of pressure. Hope this helps!