The concern in zone 5 is normally that of getting reliable bloomage. From the sound of your experience with the other mophead, there is a chance that the rebloomers ES and NB may perform as those rebloomers are intended to do. I doubt the shrubs may get 10' in places where winter kill dries out all the stems but, if the stems were not to die and the plant gets more than 6', you could do rejuvenation pruning and cut down 1/3 of the stems in three consecutive years.

One concern is that some people in zones 4-5 have had issues with ES not blooming during their whole growing season.

The landscaper's phrase 'will not survive winter' actually refers to the fact that in some cold zones like yours, the stems above the ground may dry out and die during winter (unless you winter protect). The roots should stay alive. The dead stems contain invisible flower buds developed on the prior August-September and the buds should usually open in the early Spring. But Winter in your zone will usually kill these stems & buds (unless you winter protect them).

On the other hand, since ES and NB are considered rebloomers. They should still develop new growth (stems) from the base in Spring. And when the new stems get tall/old enough, the stems should develop flower buds and these flower buds should open... in mid to late summer.

Other hydrangeas that bloom reliably and are not affected by winter kill in Z5 are the hydrangea paniculatas (white blooms that turn shades of pink) and the Annabelle-like hydrangeas. No blues or purples with those though.

If you want to try ES & NB, try "baby steps" first to see what happens. I would then buy a Nantucket Blue and observe how it performs. Fertilize once in Spring and do not prune it after the month of June. The next year, buy more NB if all goes well...

I am in zone 7. No luck with Endless Summer here. Last year I planted two Nantucket Blue. Here is one this year. So far, not much increase in size, but lots of blooms. I am happy. Not as blue, but not complaining. I can fix that.

The only macrophylla type Hydrangea that I have had bloom reliably is the lace cap Hydragea serrata, Tuff Stuff which is about 3-4’ tall when it dies back to the ground. I have tried something like a dozen other H. macrophylla and serrata including ES though not NB, and while I sometimes get bloom, it certainly isn’t reliable. In VT if they die back to the ground you won’t get bloom until late summer and into fall for the rebloomers like ES, and they won’t reach 6’; mine have never exceeded 3 1/2’. The plant is hardy, but the flower buds aren’t reliably so.

I get 100% reliable blooms from H. arborescens like Mini Mauvette and Annabelle and from H paniculata like Bobo and Quickfire, but they aren’t blue.

Your friend is probably right if you are in a genuine zone 4-5 border zone, and won’t get bloom some year. So to my mind, the questions are do you mind having no flowers some years and will you be upset with the money spent if you need to replace them, and how important is having 4’-6’ bushes since I doubt if they will reach that in VT?

Thank you for all the comments and suggestions. It is very helpful!

We are in zone 5b..Indiana..and I have ES in my 2 very large pots by our front door facing east and at the end of the porch facing NE I have ES in a very large in ground container (to keep blue better as I treat that area with aluminum sulphate). After 4 years, this being the 4th summer the end of the porch ES are full of green and about 3' tall and my 2 porch potted ES (I place them in the ground in plastic pots during the winter..called heeling in to protect and mulched well)..those 2 have bloomed consistently these past few years but not as much this year as I'd have liked. I am planing on trying a Nantucket blue and a new blue series called Let's Dance. I will not cut anything on my end of porch ES blues this fall or early spring. They may look a bit ragged but I'm curious if they will bloom. I will mulch them heavy and possibly cover with a breathable mesh. Anyone else have better luck with ES for repeat blooms? I had a NiKKo blue that I loved but hard to find locally anymore. I wonder if adding in some super phosphate would get them blooming or if that would just mess with the aluminum for the blue I want...hmm