lots!
in fact LOTS!!!
as ever with insurance it is a statistical bet.
The insurance company with make money else they wouldn't offer it.
Let's say out of £900 pa they make £300, so you are looking at the likelihood of £1800 worth of failures in 3 years.
Tesla labour is £150 ph?
Mostly the newer Teslas are actually very reliable, in spite of the endless crass reports by those with alternate agendas, but you can barely take any car into service these days without coming out with a multi £x00 bill and quite easily into 4 digits, and Tesla is a premium brand, so expect premium costs.
The one particular saving grace is the warranty is offered through Tesla, therefore any claim is handled directly by service, you don't need to get involved with claims, if the work is covered you just get a £0 bill, and trust me that lack of hassle really is worth some, in spite of the somewhat pernicious exclusions.
Lastly as I will probably sell mine privately, I reckon that it is probably worth a bit to a future owner, as I sure as heck would not touch an out of warranty Model S.
As for the gamble - well are you feeling lucky?