I have been following this thread with interest as I come from the position of having two s-pedelecs a BS10 and a BBS02 equipped Mosso 29er and my answer to the OP's original question is no......with provisos.... Firstly there is no substitute for common sense, just because you have a bike that will do 28mph doesn't mean you have to ,racing through a built up area at rush hour @28 mph is asking for trouble and even if you are using a cycle lane on the road, undertaking cars at that sort of speed is madness given the possibility of cars making a left turn. Most of my training runs are on a route 2/3 rds of which pass through a large quiet industrial estate and my average speed is just over 20 mph, however when I do rides that take in the busy "normal "roads my average is 15 mph or less ,in other words you ride to the prevailing traffic conditions. At the weekend when I use the Mosso down at the lodge I sometimes do a lovely 7 mile section of gently undulating dual carriageway that lots of roadies use for time trailing and my average speed will be up above 26 mph but this is perfectly safe and I would say appropriate place to do such a speed.
My biggest provisos would be is your 28 mph bike equipped for those assist speeds ,ie, tyres and brakes, brakes being for me the main thing, I would not consider an s-pedelec that didn't have hydraulic disc brakes, in fact I would go further and say having experienced hydraulic disc brakes I wouldn't want to buy any bike without them an example of which is that I regularly go to Lanzerote and hire a BH sport plus ( a legal Panasonic crank drive ) lightweight 17kg ,carbon forks etc and rim brakes......scary is the word only just about adequate for 15.5 mph .Whilst I was there in February I was on my usual training run from from Peurto del Carmen to Arrecife and back ,a 15 mile run on basically flat dedicated dual cycle path , I was about 1 mile into the return journey when I was overtaken by an full lycra , including proper aero helmet on a beautiful time trail bike he gave me the thumbs up and I thought I wonder .....so I accelerated up to just over 25 mph (way over the assist speed) and set off in pursuit......yeah right....he left me for dead and I mean for dead ....gone ....within half a mile I couldn't even see the guy! but I cannot tell you how scary it was even on an empty cycle path doing those speeds with rim bakes .I managed to hold my speed for a further 5 miles or so and who should I see pulled over at the side of the road having a drink but the said lycra, I stopped to chat to him ,nice guy, a pro cyclist for the Movistar team doing his winter training in Lanzerote , he showed me on strava he had averaged 31.7 mph over the last section , in his words fairly steady ! lol. I asked him how he gets on at that sort of speed with his rim brakes and he shrugged his shoulders and said he hoped for the best and that crashing was an occupational hazard, rather him than me.
To sum up I would say that 28 mph for a correctly equipped s-pedelec is acceptable providing the speed is used when it is safe and appropriate to do so.
PS. Iam fully aware that it is illegal for this level of assist speed under current legislation , I was merely commenting on the OP's question not on the rights or wrongs of the legal position.
Cheers Gaz.