Need to look at what leads to results, or else you never achieve them. Strategy 101...
I mean yeah it is not like they have a terrible defense, but it's not much, if any, better than ours. Look at the biggest difference between 2017-2018 (besides Tavares), and it is goaltending. Their goaltending went from league worse to top half of the league. Right now, Varlamov and Greiss are playing better than their norms, and if they do not, then the Islanders probably are not as successful as they are right now.
People have misconceptions about luck. Luck is probability. Nobody has a 100% chance to win, and you can have a 95% chance of winning and still lose. Without getting too technical, the idea is to maximize your chances of winning. If you are good, build your team well, etc., you increase your chances to win, and over a large sample size (i.e. the regular season), you will likely win enough to have success (i.e. make the playoffs). However, in the playoffs, there are no large sample sizes... At least in a series-by-series basis. You can have a hot/cold stretch, you could have your goalie under/over perform, you can struggle to put the puck into the net, etc... That could be the difference, even if you had worse chances of winning.
The Islanders, and I said this before, are consistent right now. They are getting good goaltending and they are converting their opportunities. Kudos to them. In a small sample size, that is enough to win even if they have a lower chance of winning due to their roster construction. However, chances are (once again not guaranteed), their "luck" will likely run out... if/when they will go up against a team like Tampa/Boston that has such a large chance of winning that it would take a lot of things to line up in order for a team like the Islanders to beat them. But all you need to do is look at what Columbus did to Tampa last year and there you go.
Ultimately, the point is that sometimes it is not roster construction or any kind of sustainable factor that is leading to success/failure, but rather an unsustainable factor. Tampa made a few tinkers, but it is not like they shook up their core and gave up on Stamkos, Kucherov and Vasilevsky when they were all brutally awful last year against Columbus. If you mistake those unsustainable factors as something more than it is, then you can end up being like the Senators in 2016-2017 and overestimate the capabilities of your team, or you can underestimate the capabilities of your team and make a rash decision when it would have just been better to stay the course. It's not something fans really think about, because all they care about is results and not how to get them... At least not to the extent that an NHL management team would need to think about it anyways.