I've chosen a modem on the basis of reliability and speed on a ridiculously long yet probably clean line. I've not managed to extract stats as doing so would be really messy and a security threat if I should make a mistake. (Off topic: It's awkward for me as I have three modems and a router configured not to allow access to the modems by devices on the main lan, but I have had many helpful suggestions as to the way to go.)
If you're modem shopping, ask people about experience with the same kind of line you have, signal quality, length, protocols etc.
In my case I have pushed very hard for ultimate speed with acceptable reliability on an ultra-long line (times three modems), and nothing else, and have not cared about manufacturer. I have done absolutely everything physically and electrically that I can think of (quite over the top) in keeping things clean to give any struggling modem the best chance to do well on an ultra-weak signal.
Ultimate speed is no use without reliability, for most applications anyway (especially as unreliability always reduces speed anyway, most-important reason being wasteful retransmissions).
In every case, over many years wherever I have read about a claimed really good modem, I have never seen any comments that the modem is good for some people’s situation and not others. Cutting corners to a ridiculous degree is rife in some electronics companies (from personal experience), but there are a few good guys out there still it seems.
Sometimes cheap kit can be really good, it's probably due to economics of suppliers’ chipset pricing too, where older less-fashionable but well-designed kit can hold its own but competition and marketing have made things cheaper. My modems are very cheap (<£20 each) but if there were anything in existence that was better then I would use it in a flash, cost no object.
