I disagree with this. For Ferrothorn to be effective (imo) requires it to run Leech Seed/Sub/Protect, leaving only one offensive move, which is probably going to be Seed Bomb. Coupled with its (somewhat) low attack, which is mostly only low because you're probably dumping all your EVs into your defenses, it's incredibly slow. This gives your opponent time to actually switch in that Hitmontop or Hydreigon to beat up your Ferrothorn (provided the other guy can break its subs), which means you're losing momentum. Additionally, Ferro isn't much of a team player; the teams I use Ferro on usually end up supporting it more than Ferro is supporting them because Ferro's moves pretty much only affect Ferrothorn. Also, Amoonguss pretty much walls it with Rage Powder unless you're using Iron Head or Thunder Wave.
This isn't to say that I don't have trouble with it when I'm playing rain teams (and sometimes sand or Trick Room). It's just that it slows your game down, and it really doesn't have room for other neat moves like Thunder Wave. It gives you a nice, tangible endgame goal (eg, 'if i can kill that chandelure, this game is as good as mine!') since its counters are fairly obvious, but it definitely isn't good for any team like Scizor and Latios tend to be.
Substitute is shit on Ferro though. I did quite fine with Protect/Seed/PWhip/TWave(toxic) on sun and Rain teams.