For one thing, and this is important, with every browser I tried, I get periodically interrupted by messages that the browser displays and that are not part of the form. Just recently, I was again asked for security updates by the browser. Sometimes when opening a form it gets prefilled by the browser, sometimes not, and sometimes only some of the fields. Some other times I get asked whether the browser should translate the form. Sometimes it asks me whether some fields should be prefilled. Sometimes when submitting a form the browser asks me whether I would like to store the form contents for later use. Sometimes the browser asks me at unpredictable times whether I would now like to restart it.
An important prerequisite for getting a lot of work done is for the system to behave completely consistently and predictably, and not to interfere with unrelated messages, questions, alarms etc. This is also very important for training new users, writing teaching material etc.
Another important point, and that is very critical as well, is latency: Just recently, I typed text into my browser, and it stalled. Only after a few moments, the text I entered appeared.
I never had such issues with COBOL applications. Today, we can barely even imagine applications that are usable in this sense, because we have gradually accepted deterioration in user interfaces that would have been completely unacceptable, even unthinkable, just a few decades ago.