While I think the replies above offer good advice, to me they don''t directly address the question of why some boat types might not do well phrf racing.
First, I think that due to the regionally different nature of phrf, that''s not a defensible generization about ANY boat.
Phrf attempts to rate a boat''s performance over an entire season in the area for which the rating applies. Naturally, there will be days where the conditions favor one type of boat significantly more than another, and even whole seasons where the race days occur on days that always favor a particular type of boat. Phrf is about average conditions, and adjusted slowly over time to observed performance of the boat types, not the crews.
If all of a region''s races occured in 6-8 knots wind and 1 foot swells with a 6 inch wind chop, it would be easy to devise ratings down to the half second for all types of boat. But if you keep two of those variable constant and start to vary say, just the windchop amount from flat up to two feet, I think you can see that different boats will be differently affected. Small light boats will get a larger hit than their rating reflects, while bigger, longer boats might not even notice the chop. On those days, the longer boats win if they race against small boats, crew work being similar. Conversely, the same boats on completely flat water are likely to have the opposite result. Similar result occur for variations in the other variables, too.
In a way, it''s a lot like the "horses for courses" analogy I read from Carroll Smith, racing engineer. Paraphrased: You can take the best race cars in the world and race them against each other, but until you know the course, you''ll never know the winner. If you match John Force''s dragster against Michael Schumacher''s Ferrari, you need to know whether the venue is the local dragstrip, Monza, or Indy. Your local phrf board has the job of issuing time on distance ratings to exactly these two cars, based on the track in their backyard. If your track is twisty, better give extra time to the dragster! Now add a Datsun 210 to the fleet, and it gets more interesting for the handicappers.