The danger is that they might enforce it if they don't like the look of you. Like if you don't pass as white enough, for example. Police selectively enforcing unreasonably low speed limits for motorists (like 15mph on an open straight rural hightway) has been a plot point in a few films over the years. Let's hope they don't make it a reality for cycling.
Which reminds me: the Cambridgeshire Busway Path has now sprouted a load of 15mph motorist speed limit (number in red circle) signs. I feared that they were planning to let drivers onto it, possibly peak direction only as it's not wide enough for two-way motoring, but no, when I checked, it seems Cambridgeshire County Council still believe the limitations in law don't apply to them and they can make motoring speed limits apply to cycling. 🙄 We discussed the wordy version of the sign previously in
https://www.cyclechat.net/threads/thats-a-lot-to-read.305566/ but it seems like nobody was obeying the sign, so they thought more signs were needed!?!?