I'm in the market to replace my 40 year old plywood bench top with a new top,  maybe a commercial laminated maple or maybe laminated MDF.

Has anyone got experience on using holdfasts on either of these tops?

Is a 1.75" maple top thick  enough for a holdfast to work?

Is a 3" MDF top too soft for a holdfast?

Thanks for your input.

I think MDF will deform around the hole within a short period of time.

Thicker the top the better for how well the holdfast will grab
1-3/4 “ might not grab that great, but better in the maple than a couple glued and screwed sheets of MDF.

If ash is still available in lumber yards it is an excellent bench top material.  Better than maple actually, in terms of how much the top will flex or sag over a span.  
When I built mine was about when there was a huge amount of ash being cut as the emerald ash borer was moving through.  So price for board ft of 8x4 was not too bad and it was plentiful.  
My top ended up around 3-3/4” thick after it was planed down and flat.

Yes, anything you are torquing on fastened to MDF will loosen.

I have nut inserts that I use for my Dillon Swage and they pulled out.  I sank them deeper, but they are still going to pull out.  I will eventually have to bolt them all the way through.

you don't want mdf. a solid core door is a cheap answer better yet search for some old bowling alley.

Rob Cosman says Industrial grade mdf works fine with holdfasts if one soaks CA glue into the expose parts of the holes.  Never tried it.  
I use 1" holdfasts in a 4” maple top with 1" counter bored out of the bottom ie 3" thick.  
I use a 2 lb Estwing drilling hammer to set and release them so they are not for delicate work.  
For that I use Microjig Dovetail clamps in the top of a moxon platform that I fasten to my bench with said beefy holdfasts.

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I used to be all about hardwood tops... Now I just go with laminated SYP construction lumber.  Cheaper and doesn't dent workpieces. Planes really easily too.  

If you're gonna do it get 2x8s and rip them in half. Then cut off the rounded edge.  If you don't have a jointer lots of glue and clamps. If you do take a light pass and assemble.  You will get sufficiently better boards this way for not much more than 2x4s

They work just fine in my 1.75”maple bench top. It’s about 10 years old and still going strong.