Some thematic, niche-focused exchange-traded funds are big winners in 2017, significantly outpacing the broader tech sector that is driving the overall market.

The outperformance isn’t solely attributed to an equity bull market lifting all boats. The SPDR S&P 500 ETF (SPY) is up 9.37 percent year-to-date as of Friday, and the Technology Select Sector SPDR Fund (XLK) is up 16.29 percent. But several thematic ETFs, such as those from Ark Investment Management and Global X Funds, are sharply higher.

The ARK Innovation ETF (ARKK) is up 43.77 percent, while the ARK Web x.0 ETF (ARKW) and ARK Industrial Innovation ETF (ARKQ) are up 40.39 percent and 30.99 percent, respectively.

The Global X Social Media ETF (SOCL) is up 30.80 percent, while the Global X Robotics & Artificial Intelligence ETF (BOTZ) has gained 26.27 percent and the Global X FinTech ETF (FINX) is 25.22 percent in the black this year.

The strength is not limited to these two fund families. Among other thematic ETFs, the Amplify Online Retail ETF (IBUY) is up 24.10 percent, while the PureFunds Video Game Tech ETF (GAMR) and KraneShares CSI China Internet ETF (KWEB) are up 37.63 percent and 40.43 percent, respectively.

Thematic ETFs are one of the fastest-growing types of funds in the space as the industry seeks new investment ideas. Some of them are actively managed, such as from Ark, and they focus on extremely niche areas. The ARKK fund, for example, invests in “disruptive innovation” companies, while SOCL invests in social media companies and IBUY holds online retailers.

But the outperformance begs the question: are they just in the right place at the right time, or do funds that focus on seeming long-term trends have staying power as viable wealth-building vehicles?

Ben Johnson, director of global ETF and passive strategies research at Morningstar, remains unconvinced of their long-term viability.

“Generally speaking, what I’ll say is for every theme there will be a season. Particular themes happen to be very much in season right now,” he says.

Regarding the Ark funds in particular, Johnson says they’ve been able to capitalize on the momentum in technology, and in the very near-term the funds have done very well.

“You can pick any of their ETFs; they look like fantastic momentum strategies,” he says. “Will that momentum persist? Who knows. Momentum has a way of mean-reverting in dramatic fashion. In the meantime they’ve stood out from the pack.”

Brett Manning, senior market analyst at Briefing.com, concurs.

“It is the perfect strategy to have on now, if you’re looking at what I would say is the disintermediation of everything that is superfluous of the way we live our lives now.  There’s a cyclical aspect to it. It’s the right time in the cycle and a few of [these funds] are the right thing for the time we live in,” he says.

Given the short lives of these funds—most are three years old or less, with only SOCL having a five-year track record—Johnson reiterates that investors shouldn’t get caught up in the near-term performance.

“If you could go back 150 years … and could create the buggy whip ETF, the buggy whip ETF would have been the SOCL ETF of the period. It’s important to understand that a lot of these thematic ETFs are fashionable by design,” he says.

It’s not that Johnson sees the concept of investing in long-term themes like e-commerce as faulty, but he points out there’s a big gap between the concept and the actual investment reality of the fund. He points to water ETFs as a prime example.

“You look at portfolios and they’re a bunch of stodgy old utility companies and maybe GE (General Electric),” he says. “There’s a huge amount of slippage between the theme, the concept and investing reality.”

Manning says he sees how an advisor might be attracted to thematic ETFs to offset ultra-safe but low-yielding holdings, but only for the right client.

“I wouldn’t pick these funds for my dad," he says. “But for my cousin who is 29, probably. These funds [Ark, Global X] look really well-managed. The implementation [of these themes] is really difficult, but I really strongly believe in a lot of these themes over the long term on a core conceptional economic basis,” he says.