IMO, racing is highly influenced by local races, teams, clinics and programs. Our racing club has a Cat 4 development team that does a great job of training racers to race as a team. They train together and have a team director. Each race they have a race plan going into the Cat 4 race. And they have done pretty well but more importantly racers have learned racing tactics and have had more fun racing.
I raced in a stage race as a Cat 5 and one of the clubs showed up with 5 racers that raced as a team and their GC designee won. This was orchestrated by the club leadership and the Cat 5 racers received expenses and etc.
As I remember, I did about 15 races as a Cat 5 that included time trials that do not count as mass start races. And it was at a time that we had some terrible crashes in the Cat 4 fields and the NCNCA decided to make the upgrade harder from 5 to 4.
I had a lot of fun racing with the Cat 5 men and by observation found there were less crashes in Cat 5 than in the larger fields of the Cat 4. At that time, Cat 5 fields were limited to 50 racers and many times we had mentors that rode with the racers. I did a crit as a Cat 5 and we rode the course flawlessly with a couple of mentors and the Cat 4 men had 4 crashes and it took them a longer time to finish.
We have races that have 45+ Cat 4/5. Those tend to be better races than the open Cat 4 since they are comprised of older racers just starting versus racing in the open 45+ or 55+ that may be too fast for newer racers.