The words are almost identical in meaning. Restrain means hold back from behind, constrain means hold back by setting barriers in the way ahead. A dog on a leash is restrained, a dog in a cage is constrained. LouiseT on EnglishForums
Startups often make progress when constrained. The most common constraint is lack of resources, usually money. But there are other common ones too: lack of experience, lack of connections, lack of knowledge, to name a few. They overlap of course.