What is the disadvantage to picking any mutual fund that has a consistent return over a ten year period?

The ten-year return is a good discriminator for choosing a fund, but is not in itself proof that future returns will match the past (as the prospectus will note). Also consider costs, the fund manager, the mutual fund house itself and any recent changes (investment style, portfolio, management) that might cause future performance to differ.

Look at the prospectus. By law, every mutual fund has to disclose risks. One of the biggest is:

Past performance is not a guarantee of future value

If a mutual fund is performing well and is consistently beating the market, you have a few risks. First, management turnover would occur if another fund wanted to acquire the fund managers. What is a fund without the people making the investment decisions? Second, consistent returns bid up the value of the fund's shares, meaning you have to pay more and more with each subsequent investment. Basically, you will have to pay a lot more per share than everyone did 10 years ago. Third, you don't know what the future will bring. It is said that 90% of returns comes from the asset allocation decision. Let's say that tomorrow the energy sector takes a long-term dive. If the fund is heavily invested in the energy sector, you would lose. So while it may less difficult to pick industries than securities within the industries, if the allocation decision is wrong moving forward, then all of the positive history they have does not count for anything.

A fund that has had great returns consistently over the last ten years may be GREAT, but it may also just have had its money in the right place for the LAST ten years, which may or may not be the right place to be for the next few years!