Yes, it's not easy to compare road race times with other road race times.
I guess thats why they used to be world best's, rather than world records.
Having been out & measured road races with a qualified course marker, I know it's very unlikely any of us are running under distance.
When they measure, they really do take the shortest line, they will ride on the opposite side, into oncoming traffic, so we get to race whatever distance +
So this leaves the question of downhill races, I'm not sure what the rules are here but the FLM gets away with it's down hill route, although you do start by running uphill.
The Lisbon 1/2 doesn't count as it's point - point & downhill.
So it's hard for us to know whether our local road races are for PB purposes!!!
So it comes down to the individual I guess.
I would not use a race that is point - point & downhill as my PB!
Having said that
My 10K PB is 33:34 but on an undulating course & down hill finish, I've also run 33:42 on the pancake flat Cardiff 10K.
So I'm happy to call my Ashford 10K a PB, the same course Charlotte Dale has just race in 32:14! to go into the top 5 UK womens times.
My 10M PB is 56:00 on a flat course, but I've also run 56:01 & 56:03, of those three performances the 56:03 is the better time as it's a hilly course!
My 20M PB is 2:00:12 in the Spen 20M, which has a difficulty rating by the BARR of 3min brining it down to 1:57:12, but thats not my PB!
And those 12sec hurt!!!
My marathon PB is 2:47 but my 2:49 in Madrid is still a better race effort as it's hilly & at altitude [800m]
Virtual racing would give me a 2:36 marathon, but I've to prove it to myself in real time & effort.
One of my best performances came in training :-)
Dropping MikeG on a training run in Davos, he went very quite afterwards :-)
Only can only cheat yourselves I think!