I actually went from ER collets back to endmill holders because I couldn't get our collets to run true enough or hold tight enough for our applications. I suspect the junk Chinese collets I was supplied with had something to do with it, as well as the fact that I could only get metric collets that didn't line up very well with our imperial tooling. Side lock EM holders will always have some run out, so ER collets have the potential to run more true, but I was never able to get even close. I don't think "standardizing to only one tool" is ever a good solution, I like to pick the best tool for the job. And ER collets are a very good choice for many jobs. But for high pull out forces on endmills or very tight clearance areas, I've found normal sidelock holders to be superior. On the other hand, tap holder collets are a godsend, and having a good ER selection for oddball drill sizes and weird form tools is flat out necessary.