We don't tend to get the packs of mass cyclists around here like in London, etc. but it is increasingly getting frustrating during the summer months as you end up with more and more strung out with few good places to pass them, along with a seeming ever increase in motor vehicles on the road, adding 20 odd minutes to a journey and you no sooner get past one than you catch up the next leading to increasing frustration and people doing daft things to overtake them. If it continues like this it isn't really a sustainable situation and has lead to a notable up-tick in the last few months in cyclists being killed on the roads around here. (I suspect partly due to more people taking up cycling with the whole COVID situation).
Other frustrating thing is visibility - some take it seriously but far too many don't seem to have a clue as to how they look from a driver's perspective. High vis really can add seconds to how quickly a driver notices they are there, or notices at all...
A flashing red light does a great job of making the cyclist visible early but without the addition of a constant rear light makes it harder to judge the actual position, heading and speed of the cyclist. Having ankle/shin level bright or better reflective markings do a great job of making the cyclist visible especially as they are constantly in motion but should their rear light fail them for whatever reason the lack of high vis higher up could make a huge difference in being noticed.
Driving a lot at dusk/night I've had a scary number of encounters with cyclists who are only visible at any real distance because a thin line of white sock material was visible going round and around.
EDIT: One from the other day:
From what I could see the front cyclist only had a forward face light, the rear cyclist had a bright coloured top on but I couldn't see any lights on the bike - I'm not sure if the other driver purely made a bad decision on going past them or additionally reacted late due to poor visibility on the part of the cyclists.