When sending a fax using a class 1 modem, the software has a lot to do with how successful you will be at completing faxes to arbitrary fax machines. The modems themselves are not equal, and the fax machines you are sending to certainly are not.
A good quality fax sender implementation will give you more successful faxes than a poor quality one.
But if you move to brooktrout - where - as I understand it - all the intelligence is on the fax board (is this right?), then assuming you can get your document converted to a TIFF and it starts to send, are you going to see the same success rates from every type of fax server software? --- In other words, is it the brooktrout board which is responsible for the success or not of a transmission, rather than the choice of fax server software?