You can learn something in any kitchen, even chains. In fact, theres a lot you can pick up from working in a chain restaurant that does mostly heat-and-serve type cooking.
The big things are organization, timing, cleanliness, and speed. And those are very big things. A lot of times these things are more important in a chain than in a stand alone, from scratch kitchen. When its 4 guys on a line doing 500 covers on an 80 item menu, each and every night, with a KM over your shoulder yelling about 7 minute ticket times, there is no room for error, and those highly successful chains won't carry any deadweight for long, so if you can't keep up you're gone. So you'll learn a whole bucnh, or you won't last.
Unfortunately, theres a lot you won't learn as well, as the breadth of actual cooking in those places is slim. Generally those places aren't making their own stocks or soups or breaking down primals or hand making pasta or desserts or bread, etc etc etc.
But in my experience, I'll look to hire somebody with solid chain store experience over someone fresh out of culinary school. If I see a guy spent two or three years at Olive Garden, and he checks out when I call about it, I know that person is a proven cook. Someone out of culinary school with no experience is unproven (and in my experience has a 50/50 chance of being worthless). I don't care if you're a CIA grad...can you hold down a fry station on a busy friday night? Because if you can't at least do that, I've got no use for you, no matter how many different sauces you have memorized. I can train someone to make the things I need made, but its hard to train someone to be fast, organized, and clean short of just yelling at them. So I wouldn't look at having mostly chain experience as a handicap by any means
But, that experience will only get you so far. And if you do find yourself in a scratch kitchen, you'll probably be behind the learning curve a bit. There is a very big difference in the work needing done, prep work mostly. The amount of prep needed in even a smaller kitchen can be shocking to someone coming from a heat and serve background. Some things take hours and hours to prep, or even days. But with a background in chain stores, and being clean and organized, and being comfortable in a kitchen and using kitchen equipment, it will come quite easily to you.