You may want to consider the better BBU ( the FTTP one ) , if on copper , the BBU only needs to power the router ( presumably whatever device you use already has a battery, like IPad or laptop) , if you were on FTTP the BBU needs to power the router and the ONT , so although you don’t have an ONT now , because you are not on FTTP , it would future proof yourself for when you do get FTTP .
As far as real world experience, the power cut ‘panic’ is so overblown ( IMHO ) that the chances of those that claim they needed a BBU , and got one , and then have actually had to use it in a real world situation ( during a power outage ) will be small , so getting that small group ( BBU and genuinely needed it as they had a power cut ) to comment on here will be unlikely, but you never know, some may comment but wether they genuinely had a real world outage may be up for debate.
As the BBU takes over the power supply of the router if the mains fail , for as long as the battery lasts in the BBU , the broadband service will be identical ( the router doesn’t care ) with the small chance that as the mains power fails and the BBU kicks in , the router may briefly drop out and have to re sync ( a small break in service ) the way these work is your router is connected to the BBU , the BBU connected to the mains , as the mains fail the BBU takes over .
