The advantage of BRT is that you can combine segments of a route that have a dedicated lane with segments that don't. Counting operating and ongoing capital costs, the dedicated lane bits aren't really much cheaper than rail, but in theory you can combine express segments with local ones so that buses can make use of existing roads and also the dedicated lanes. Rail obviously can only use the bits you build for it.
But the problem with BRT is always BRT creep. People start deciding that they can save a little money here and there, and that dedicated lanes aren't such a great idea after all, and then you have... a bus.
And buses are great, but they're buses, stuck in traffic like everybody else.