It's highly unlikely to fail an MOT if you fit decent quality LED units. HID will definitely fail but LED will be fine. It's my Facebook group that you might be referring to, and not one of the members has had their motor fail MOT based on the LEDs that they retrofit. We pretty much all use the same guy and his units are plug and play, no errors and improve the field of view dramatically.
An MOT testwr is not allowed to remove the cap at the back of the light to see what lamp is in there, therefore they can't be sure. And if they can't be sure then as per the handbook they must pass the vehicle giving it the benefit of the doubt.
I will say that they can and will fail you if you install cheap ones that have a poor beam pattern, or if they dazzle oncoming vehicles. The ones we recommend do not do either of those things. You may need to take it to a garage once you fit them and just get the alignment checked to be safe though.
His main business page is Facebook, but I appreciate you don't use that platform. He also has an eBay page that you can contact him on. Drop him a message, his name is Andy, tell him Jon the admin from Mk5 Mondeo group sent you his way and he'll look after you. He does all the LEDs you might want, side, dipped, main, fog, puddle, side mirror repeaters and number plate lamps.
**I receive no financial benefit for putting you his way, I just know how dangerous and woeful the lights are as standard**
https://www.ebay.co.uk/usr/hna-supplies