Well, I never said the Tyan was better, I said that I prefer it. I would need way more information before I could say something like one is better than the other.
The IDE controller for the EPOX is an add-on chip, whereas the Tyan uses a newer 686 Southbridge and does not need it, so the data transfer rates can differ. It could also have been a driver issue, a lot of times that is what causes big differences with data transfer. Not sure though, I hate IDE and always use SCSI.
As far as problems with the video cards, I have had none at all, but maybe because I am not using the same ones you are. I only buy ATI, because I never have had problems with them (except for the initial AIW Radeon, which works fine with the drivers that come with it, but goes nuts if you use newer drivers. So much for progress

One problem that you may have had is an old MVP3, the CD version, which caused all sorts of problems. Unfortunately, Tyan used these chipsets way too long even after it was discovered they were problematic and the CE version was out. Luckily, the ones I got were all CE version so are not a problem, but if yours were CD versions I suspect you would have far more problems than someone with a CE. I suspect your Epox is a CE, mine is.
The newer revisions of the MVP3G5 may be able to run at 550 MHz fine, but mine will not. The newer S1598C2 may run with fewer problems than the ones you have (or maybe you have a new one), so it may just be the revisions of the motherboards more than the companies themselves. Having said that, Tyan using the CD version of the chipset for so long is certainly not something they should be proud of, so if you got stuck with one of these lemons they deserve your acrimony. They should never have been used after they had better choices.
If you get a chance to look, or remember, let me know what revision your chipset was. It would be interesting to see if that is the cause of a lot of the problems.