The thing is, Festina didn't have an absolutely top-class rider to win GTs until they signed Zülle in 1998. Virenque was a very good climber, but until 1997 or so he lost a lot in the time trials. Few people thought he could improve on his 1997 podium, and that was before we knew just how juiced he was. Dufaux was a decent time-trialer, but he didn't climb as well as Virenque. Riis's program was probably superior in 1996 because he apparently wanted to die or something, but in subsequent years Festina was the most powerful team. They just lacked someone to win.
Zülle should have been that someone. However, in the 1998 Giro they went to such ridiculous lengths with their program that it backfired in the last week. I read somewhere that Zülle wanted the same Dufaux tried at Romandie, but a week-long race is not the same as a three-week race.
I have no doubts that Festina would still have been the strongest team at the 1998 Tour, and at least one of Virenque, Dufaux and Zülle would have been on the podium. And if Zülle hadn't already done the Giro I'd say he could well have won the Tour. But Virenque and Dufaux weren't Pantani and Ullrich. Their program was pretty ridiculous though, it's up there in the list of team-wide doping's greatest hits together with Gewiss, the good times of Mapei, ONCE from 1995 on, US Postal/Discovery or Saunier Duval (edit: or Casino!)