Flying to Europe hasn't been this cheap in years. But before you go ahead and buy that $140 one-way ticket from Boston to London, be aware of all the drawbacks, such as high fees and lots of restrictions.

The dirt-cheap fares are the result of ultra-low-cost airlines that have cropped up in recent years. They include Norwegian Air, British-owned Condor, Icelandic carrier Wow Air, France-based XL Air, and the Latvian Primera Air.

The deals are certainly enticing. For example, a one-way fare on Wow Air from Los Angeles to Barcelona in December can be had for $199, according to its site. By comparison, a search on the flight aggregator found a one-way flight on that route on Iberia, a Spanish airline, for $470.  

And that's just for starters.

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Condor, which flies from New Orleans among other cities, is offering one-way airfare from Austin, Texas, to Berlin in November for $569. The cheapest fare for that route in November will run you about $700 on Condor also partners with Alaska Air from cities like St. Louis. 

Primera Air, which begins service in 2018 is offering one-way flights from Boston to London for $140.

Norwegian Air is offering a one-way fare from New York to Dublin for $139 in December, according to its website. Kayak's cheapest fare was $397.

So that's the good news.  Now, here are all the reasons why you might want to think twice before jumping on these fares.

$130 for a Checked Bag

Ultra-low-cost carriers generally charge extra for just about everything.

Checked bags, for instance, can be very expensive. Norwegian charges $130 for each one.

You'll probably have to pay for food and drinks as well. Wow, for example, sells a small cheese pizza for just under $10.

There are exceptions. XL says its fares include food and one checked bag. The carrier flies from San Francisco, L.A., New York, and Miami to Paris, Tel Aviv, Guadalupe, Martinique, and Reunion, off the coast of Madagascar.

Wow Air's charges depend on your route. For flights from New York to London, for example, checked bag fees range from $59 to $99 depending on whether you made the decision online or at the airport. Carry-on bag fees range from $39 to $99. Wow allows one personal item (purse or briefcase) on board for no additional charge per passenger.

Even with all the extra charges, you'll still end up paying less than regular airfare. But there are other things to consider. 

Out-of-the-Way Airports

Flights on some of these ultra-low-cost airlines aren't always direct, and they often take off or land from less conveniently located airports.

In the New York area, for example, Norwegian departs from Stewart Airport in Orange County, New York, about an hour and a half from Manhattan on a shuttle bus that costs $20 per adult. It also has New York area flights departing from Newark Liberty International Airport.

If you fly Wow Air, you'll have to make a stopover in Reykjavik, Iceland, before continuing on.

On flights to London, Wow lands in Gatwick, a 30-minute, $24 (per adult) express train ride to Victoria station in London proper, and in Stansted Airport, which is a 40-minute, $9 train ride from downtown. Meanwhile, Heathrow Airport is a 15-minute, $35 (per adult) express train ride to London's Paddington Station.

Norwegian also lands in Gatwick, making many connections there. But remember, if you’re connecting with another airline, make sure it departs from the same airport or that you have enough time to make your connection at another airport.

Delayed or Canceled Flights

One of the biggest complaints about ultra-low-fare carriers is that they tend to have more delays and canceled flights, according to industry experts.

"There are inherent risks when you choose to fly on a smaller airline, since these carriers tend to have fewer resources—such as spare aircraft and crews—when schedules are disrupted and there are lengthy delays or flight cancellations," says William J. McGee, aviation adviser for Consumer Reports.

If the airline cancels your flight, you have a few options.

"If we cancel due to schedule changes then we must let our passengers know with at least 14 days’ notice," said Wow Air spokesperson, María Margrét Jóhannsdóttir. "In those cases we offer passengers the possibility to move their flights to the next scheduled flight, get a refund or change to another location," she said.

"If the cancellation is due to another reason, such as irregular operations, where we have, for example, passengers that have already checked in or are just about to, then we offer three options: a full refund and cancel travel; change booking to next available flight, or change booking to a later date of their choice," Jóhannsdóttir said.

Wow Air flies from Reykjavik to Pittsburgh four times a week, so if your Wednesday flight is canceled you'd likely have to wait until Friday. But if that flight is full, you’ll have to wait until the next flight with available seats.

Condor says that since it only flies a few times a week it generally would delay—but not cancel—a flight.

Susanne Rihm, a spokesperson for Condor, says less than 1 percent of its flights experience delays of more than 3 hours. Norwegian and XL didn't respond immediately to a request for comment on this story.

Travelers with tight schedules might feel more comfortable with carriers offering more regular service, Hobica says.

"If your British Airways flight from London to Seattle is canceled, you’ll likely be rebooked on another flight just a few hours later," he said. “If you’re worried about missing an important meeting, which airline would you choose?”  

Still, no matter what airline you're flying, there are certain protections for stranded passengers flying a European airline or on routes from, to, and within the European Union, McGee said.

"[Travelers] are protected by EU's passenger rights regulations that mandate compensation and assistance. These are the same basic rights Consumer Reports is fighting for here in the United States," he said.

Those rights protect you if you're stranded anywhere in the world. 

"If you're flying from L.A. to Tahiti on Air France, an actual route you can take, and you are stranded there, the airline will have to pay for your accommodations, food, and phone calls under the EU rules," Hobica said.

When You Cancel or Miss a Flight

Tickets on ultra-low-cost airlines are nonrefundable, but they'll allow you to cancel a reservation within 24 hours of purchase if the flight is 7 days or more away.

However, you won't be refunded or rebooked if you have to cancel your reservation at the last minute or miss your flight.

Wow and some of the other low-fare airlines may offer cancellation insurance in case you become ill or are hit with an unforeseen emergency at the last minute, or they may make allowances if you purchased premium tickets.

It's important that you understand what emergencies are covered because some airlines define them narrowly. Also, make sure to check with your credit card since it may offer travel insurance that can help in some circumstances.

Who These Flights Are For

George Hobica, founder of AirfareWatchdog, a popular fare tracker, says that these cheap flights are especially attractive to younger millennial flyers who generally don’t have a family and don’t care about perks. They also tend to be more flexible with their travel than families or business travelers.

This cohort tends to travel light and doesn't usually check bags. Reserving a specific seat is usually an extra charge on these flights but, for the most part, Hobica says, millennials don’t care where they sit. 

“For them there’s never been glamor to flying and they don’t care,” he said. “They don’t miss what they didn’t have.”

Clarification: This story has been updated to clarify which cities Condor flies from directly and which cities it partners with other airlines.