I'm an instructor with significant time on a range of interesting aeroplanes, so it's not all that unusual for me to be asked to conduct checkouts for owners, usually syndicates with new members. I make a point of working out who matters - owner / syndicate chairman / etc. and asking for a very clear statement from them of what standards they want to see. Some are happy with "average PPL", some want a lot better, a few are really not all that worried. That is absolutely their call, but it's important that I know the answer - if the standard is to be significantly higher than PPL pass standard, I've no problem with that, but it's important that everybody knows.
An interesting recent experience was a bizjet Captain with about four times my hours, who bought a share in a vintage Aeronca. He really struggled with the handling of the aeroplane, but had a great learning attitude It took about 10hrs, and he was totally happy and relaxed about it - a learning attitude like that is an absolute pleasure to fly with. He's still flying the aeroplane, and very safely. I have had less experienced pilots who also struggled, and got very unhappy or even aggressive about it - that can be a lot more of an issue.
G