It depends on which components get mixed with which. For example, one brand's shifters sometimes aren't compatible with the other's derailleurs. Also watch out

It depends on which components get mixed with which. For example, one brand's shifters sometimes aren't compatible with the other's derailleurs. Also watch out for chain widths relative to cassette gear spacing (more a concern since 2x10-speeds hit the market). But for the most part, we're talking about mechanical (not hydraulic) devices that can be tuned/adjusted to work with each other.