The accumulator is attractive because small odds become a big one. Four legs at 1.50 give 5.06. The same multiplication applies to the risk.
Probabilities multiply
For an accumulator to win, every leg has to win. If each has a 70% chance and the legs do not affect each other, the chance that all win is the product.
The margin multiplies too
Every leg contains the bookmaker's margin. In an accumulator the margins multiply. If each leg carries a 5% margin, the slip pays only (1 / 1.05)^N of the fair payout.
When it makes sense
It is not forbidden. If you play a small amount for fun, an accumulator is a reasonable choice at the cost shown above. If you want long-term profit, single selections with positive EV, or accumulators with a few well-chosen legs, give away less in margin.
Watch correlation as well. Legs from the same match are not independent, and bookmakers price that into bet builders. See our combinations in the bet builders.