Warframe is really confusing to me. I keep seeing it being praised like you just did, when you hear about it, it sounds like a really high quality game, but on the other hand I also always hear about the insane amount of grind in it, so it seems paradoxal and confuses me.
It being a free to play, with apparently ton of grind just sends me all the bad signals, because... Well when you have grind in a game, it's usually either to hide the lack of content, or in online games to frustrate players into purchasing things to go faster, which doesn't sound like something the publisher would do if they are like what you described.
I don't know, I must be missing something, but everything I hear about this game seems to be in contradiction all the time, it's very strange.
I only played it a bit a long time ago, back when there wasn't the open world and all yet, it was very repetitive which is what made me stop, but it seems to be a lot more complete today, but at the same time it still seems to involve the same amount of grind as before