I've used it in NB for a year. Coverage is pretty good. Eastlink's own service only uses those spectrums, but they seem to perform well. I drove all the way to Ontario and back with my Eastlink phone and stuff like streaming music never dropped.
Outside of their area, you just roam on one of the national carrier networks and it works fine. The only issues I've had are if you're right at the edge it may try to use the Eastlink tower when it shouldn't, and that you can't turn roaming off in areas that don't have coverage so you need to be careful near the US Border. In Saint Andrews, NB, for example, on the beach you can accidentally hit a US tower instead.
I have 3 phones in NL using it since Dec of 2018, can't say that I have had a problem with the service in NL. I travel a bit and notice some differences while in the US, Europe or the Caribbean vs my previous Telus.