Simply: They play different..."bad" different...so while it may work in the short term because they're playing "wrong" and do unexpected things...in the long term, they do not have the fundamental basis in which to improve upon and, too, when teams actually need to figure them out, they usually do because these type of styles are wrought with holes...
Teams that only play these guys every so often, generally won't put in an excessive amount of work on figuring out these flaws...when Calgary comes to town once a year to play Boston or whatever, they're probably just going to go play...it's like a knuckleball pitcher, you can spend all your time learning to hit this guy or you can just go out and play and wait for a normal guy...if you get a hit, you get a hit...if not, well, at least you didn't throw yourself off for weeks on end...
Teams you play most often, particularly rivals, will spend the time to figure these players out...that's why Montreal and Toronto often torched Thomas, just like even the lowly Penguins (at the time) torched Cechmanek...
And to that point, both were involuntarily jettisoned from the league due to their poor play after just a couple years each of relevancy...