I’ve spent a lot of time on motorways moving buses between Sussex and Wales. It’s not a big deal if a bus can’t go at warp speed because there are usually at least two overtaking lanes for lorries and other vehicles to use. There's also a lot of car drivers who aren't confident doing more than 50mph. If a bus can't do at least 50mph however then it shouldn't really be allocated to motorway work. With engines packed tightly into compartments, cooling capacity is limited on many buses so doing 62 or 70mph as a matter of course isn't feasible. Lorries and coaches have a lot more room to facilitate airflow and bigger cooling equipment.
My Citaros all do 59mph, which is what Mercedes set the legal limit to. The only exception was a bendy bus which could be set to anything up to 87mph, but I set the limiter to 60mph to keep it legal. Citaros of all kinds are exceptionally study vehicles which hold the road well and ride smoothly at speed. They also have enormous radiators and either a pair of huge hydraulic fans or eight powerful electric fans. They can sit at 59mph for 180 miles without so much as breaking a sweat.
Not all low floor buses are capable of that. My 2005 Volvo B7RLEs were fine at 62mph as was a 2007 Optare Tempo. Dennis Darts should not exceed 43mph under any circumstance. Double deckers also have to be watched. Volvo B7TLs and B9TLs don't like exceeding 50mph for more than short bursts and doing high speed work with those with electric fan retrofits is not advisable. By 53-56mph, the stability of the oil temperatures takes a tumble and the lot will get hotter and hotter until you slow down. E400Hs I understand consume more energy than the generator produces if they go too fast, resulting in them losing power then grinding to a halt. You then have to idle for a few minutes to get them moving again. This is why they're limited to around 45mph. I've no data on how electric buses cope with motorway work.
Despite being told by many people to buy E200s and E400s due to lower operating costs and simpler maintenance, I understand the nature of the work I do and I choose the vehicles accordingly. I target buses with a 6 speed gearbox for purchase, along with heavyweight single deckers for this reason. The way I see it, it's not cheaper to subject buses to heavy going work that exceeds their design when you have breakdowns which not only cost money to deal with, but result in lost revenue and a hit to reputation. Citaros may not be the cheapest buses to run, but I can't fault them for reliability and versatility.