I am not really talking about cross-language dependencies. I mean simple things like Java product engineers who know gradle and want to use gradle and get grump

I am not really talking about cross-language dependencies. I mean simple things like Java product engineers who know gradle and want to use gradle and get grumpy when they don't know how to add dependencies to WORKSPACE. These people need to be trained to use different tools rather than use the skills they already know. That's fine, but it's definitely a cost to using bazel. Or if you are trying to maintain bazel for an org. Invariably, you will receive requests of the form "I would like to achieve [well document thing in language-specific build tool], please help me do that in Bazel". The net result for me, who was once in that role, is that understanding the original build system, then bending bazel to match a certain behavior (usually with no or inadequate docs) is a huge amount of effort. Enough that I now am skeptical of the value-add of bazel.