5e Changelings have basically no drawbacks. Can turn into any medium humanoid creature, no saving throw concentration on holding a disguise if you take damage, and the disguise is considered perfect unless you do something REALLY out of character.
Weaknesses. Connection to their mother – Killing a mother changeling will cause its children to die instantly. Fire – Setting a changeling on fire will kill it. Mirrors – A changeling's true form is revealed in a mirror.
WotC has confirmed that changelings are considered to have the Shapechanger subtype. So they are indeed immune to polymorph and can be affected by moonbeam.
Most changelings don't know they're changelings.
Though they may always feel somewhat different or removed from human life, most changelings are too young to understand who they truly are.
Personality. Changelings are commonly harmless, passive people and are uninterested in politics and social affairs.
Changelings born from two changeling parents are born in their natural form: pale, slender, with colorless eyes and white hair.
Changelings don't inherently recognize each other. If they don't want to be recognized then when they do something suspicious it would be an insight check against the changelings deception, which the changeling has advantage on. A changeling could have expertise in insight from divergent persona.
Changeling blood is green, unlike most other animals. Changelings, despite their thinly and seemingly fragile frame, are capable of surviving otherwise fatal injuries thanks to their magical abilities.
The short version is that a Changeling can shapeshift to a form that can produce sperm to impregnate someone, and can shapeshift to a form with a uterus and eggs to get pregnant, but a pregnant Changeling cannot shapechange to a form without a uterus without ending the pregnancy.
You can't increase or decrease the number of limbs you have, and their basic arrangement needs to be in roughly the same location on your body (2 near the head and 2 near the rear). So, you can't, become a winged demon or angel, as the wings of those forms are additional limbs.
By RAW changelings revert when they DIE but not when they are unconscious (it would be hard to maintain a long term masquerade if you revert when you sleep!). I bet suggested that a piece of a Changeling — such as cut hair — WILL revert, but that this is slow and may take up to 24 hours.
Some stories tell of changelings who forget they are not human and proceed to live a human life. Changelings who do not forget, however, in some stories return to their fairy family, possibly leaving the human family without warning. The human child that was taken may often stay with the fairy family forever.
Being a changeling means you feel as though you came from somewhere else, and will go somewhere else. It means acknowledging that you are here temporarily. It means seeing the magic in the world, and if you can, representing it for others, so perhaps they will see it as well.
Appearance. In their true form, or their form "at rest", changelings have pale skin, colorless eyes that can be completely white or completely black, and white or silver-white hair.
A changeling's true form is revealed in a mirror. Fire - Setting a changeling on fire will kill it.
If the changeling has lost a limb, the limb is missing in every form they take. If they haven't lost a limb, every form they take has all the limbs the changeling normally has.
Changelings typically have large eyes with narrow pupils, ranging in color from white, to yellow, to black. Hair colors are similar: Light grey, platinum blonde, shades of silver, all the way to the darkest hair color being a light brown.
Changeling Traits
Age: Changelings reach adulthood at the age of 16. Their fey heritage allows them to live much longer than humans, up to 200 years.
Changelings have red blood like a majority of the other races. Bleeding around others may alert many that the Changeling is not of the same race (should the blood color vary). For this reason, a Changeling will usually choose to take the form of a race with red blood.
Vocal Replication: All Changelings can mimic any sound they hears using their voice. Voice Manipulation: All Changelings can freely manipulate their voice, allowing them to control their voices to imitate sounds of creatures such as animal noises and explosions or increase or decrease the tone of their voice.
Some types of octopus, squid, and crustaceans have blue blood. Their blood contains a high concentration of copper. When copper mixes with oxygen, it gives their blood its blue color. Blue and red are not the only possible colors of blood — some animals bleed green.
Color. The color of human blood ranges from bright red when oxygenated to a darker red when deoxygenated.
Unlike doppelgangers, which are entirely genderless, a changeling does have a default gender that manifests in adolescence, but each changeling can adapt his or her form to be of either gender, hermaphroditic (both genders simultaneously), or entirely genderless.
Changeling Instincts: You gain proficiency with two of the following skills of your choice: Deception, Insight, Intimidation, and Persuasion. Languages: You can speak, read, and write Common and two other languages of your choice.
Changelings mature slightly faster than humans but share a similar lifespan—typically a century or less. While a changeling can transform to conceal their age, the effects of aging affect them similarly to humans.