Why are some people allergic to favs?

Never make such rigid restrictions on your betting but on the whole, I find backing favourites is a losing strategy for me. That does not mean that I won't back one if I think it is value. The worst thing to do is back a favourite just because it is the favourite.

Suppose everyone has an allergy of some sort , with me it's potatoes & bananas

For me it is yeast unfortunately. Draught Beer and freshly baked bread are very bad news. It took me a few migraines and acid indigestion to realise.

That fav at Yarmouth went to 20 in-play, and still won

I saw some stats a while ago which shows that favs on average only have a 30% strike rate and over a period of a year would loose about 300 points at LSP.  I suppose the strike rate of horses over 4/1 is the key as you can afford a 25% strike rate and still make a healthy profit.

@desp You meant NATURAL PATH (IRE)? He traded at 75 IR. Favs never lose.

@Donb Everyone knows that, but the strike rate is obviously dependent on BSP. So many people think that anything below 2/1 is poor value even though half of them are certs.

Anyone who thinks favs are poor value should try laying them, then you'll see how worthless the fields really are. If you had blindly backed 4977 favs since January 2020 at odds 2-3 you'd be down -148 pts with 5% commission, which is only -3% ROI. Most punters who study racecards for years have worse ROI than that.