BMR units usually have restrictions on their resale prices so that they are not inflated by the sleazy real-estate speculation process that artificially inflates the price of "market rate" housing and creates disastrous boom and bust cycles, bubbles, etc. The resale restriction also ensures that the subsidy money put into the unit (to enable it to be sold below market rate) keeps it affordable not just for the first buyer, but for generations of buyers that come later.
As a buyer that means you get a good price (if the market value was $300,000 and $80,000 of subsidy were used, you get to live in a $300,000 house for $220,000), and you get to act as a steward of an important community resource (affordable housing).
When you are ready to sell, if you are in a rising market you don't get to take out value that you didn't put in (unlike like most homeowners around you), if your market is falling however, it's less likely that your price will fall because it was less likely to be artificially inflated by speculation in the first place.
One of the biggest problems in the recent housing crash was that people bought houses for say $250,000 and then when the market price rose, due to speculation, they refinanced and took out loans against the new value of say $500,000. When the market started coming to its senses the bubble burst, home values started to reset to more reasonable levels based on supply and true demand (rather than speculation) and lots of people ended up owing more than their houses were worth. Resale restrictions, when done right, should prevent you from taking out equity that you have not put in and will help you avoid this problem in the next crash.
Research on resale restricted low-income and middle-income housing has shown that it gives people the stability to create a good credit history, build savings and equity and it holds its value in bad times. It is a safe place to put your money, and will probably provide a higher quality home than similarly priced units that are not resale restricted (because they have not been subsidized) but like all truly safe places to put your money you won't make a big profit.
If you get to the point where you have the income to afford a market rate house you will be able to take out the equity that you put in to the BMR unit and pass the affordability on to the next person who needs it (and I don't see that pool shrinking any time soon).
If, in the future, you have reasonable credit, have saved enough for a down payment and have sufficient income to afford a market rate mortgage you will be able to buy a market rate house. Living in a BMR unit can help with the first 2 of those, the rest is up to you.