1) that the business case doesn't really scale, and the economics mean they're only able to infill a few marginal notspots. I also note that they are using BDUK funding - does it stack up without subsidies?
2) They make a go of it, but some Big Scumbag Corporation (name your choice) buy them out, and it all goes to pot.
3) The start to make a go of it, but then a Big Scumbag Corporation (only one name in here) start targeting rural roll out to intentionally nobble them, whilst Ofcom sit and do nothing.
My money's on number 3.
In the village I'm in, GC deployed FTTP just as OpenReach showed up to deploy FTTC - but the plans for the local exchange & cabinets had been known about for quite a while, so it's not news to GC that BT are upgrading cabinets in areas they are deploying into. BT will find it difficult to compete on bitrate, or even price. My money's on #2.
If they go the #1 route, then when the subsidy runs out either they raise prices significantly and customers leggit (although their T&Cs limit price increases to 2% above RPI), or they go to the wall, in which case #2 will happen.