Both of my own longest running 5e characters have ended up as Lycanthropes (one by complete accident when nobody involved understood the consequences, one semi-purposely), and I briefly DMed for a character who did as well. It has been fun, but it is definitely not a well-balanced aspect of the system.
The first incident was a chaotic (mostly) good character bitten by a wererat. Neither I nor the DM really wanted to enforce the alignment change to the exact opposite alignment, so instead we experimented with a system where I had poor control over the transformations, and my character would go to sleep and the inn and wake up naked outside town smelling of cheese, and such things, which led to some fun shenanagans. In order to master the transformations I had to eventually accept only being resistant to nonmagical, nonsilvered weapons damage instead of immune. During the immune period there were only a couple solo, RP-oriented fights that wererat immunites completely trivialized, and only three or four other real encounters where my character was able to operate with impunity, which felt like a good balance of enjoying a power-trip without unbalancing the overall campaign. This all worked out reasonably well but only because the DM usually remembered to include things that could hurt me in encounters and I was a compliant player not wanting to radically unbalance the game. If you have a "lets all become wererats" type of player and group its a lot more unbalancing. Also that group's tanky frontliners all had players who came irregularly, so having my character, who as a sword bard was competent in melee, but middling in the HP or AC departments, sometimes be semi-invincible made it possible for us to still adventure on some days when we otherwise would have had serious trouble doing so.
I'm currently playing a werebear Fighter/Barbarian in a very rules as written campaign based on a published module. I would say it has been quite a bit more disruptive here. Firstly because this character was the group tank anyway, so upgrading their defensiveness has greater consequences for the group's overall capabilities than my other character. Secondly because we are still playing many encounters as written and so sometimes, particularly in horde of low level enemy fights, it just turns out I am immune to everything on the field. We did enforce the alignment change to werebear neutral good, but my character was already true neutral leaning a little to the good side, so while it made for some interesting RP it wasn't a major limitation. Fortunately the rest of the group has solid character reasons to not want to embrace werebear lycanthropy, most encounters are still plenty hard for our four man group, and while my character is fairly invincible sometimes, most times there are still things that can hurt him plenty.
The character I DMed for purposefully became an Aarakocra wererat by having his character stick his hand into a captured, unconscious wererat's mouth and rolling for it. The player really embraced the idea of sudden evil tendencies, and did so in a way that forced that character (whom he was tired of anyway) to have to part ways with the group. We only had one major combat while he was a wererat and still with the group so the immunities only ever came up in a solo fight he had with the guards while escaping jail and abandoning the party. I had intended to have him only be resistant instead of immune, but I forgot in the moment and the character was being retired so it didn't matter.
My general advice would be to go with unpredictable transformations at first and probably only resistance to weapon damages at first and see where that gets you, the player, and the table. I would definitely make the curse not be immediately contagious from them so that you don't have a whole group of lycanthropes. I might make them sensitive to silver. I think "take the character away, they're a monster now" might make sense if you are wrangling a bunch of 12 year-olds (or players who act like it), but I wouldn't do it with most mature players. I don't think alignment enforcement as a cost for mechanical advantages is a good fit for 5e, so while I might play around with the alignment aspects a little I wouldn't really enforce it. I would make a point of remembering the characters immunities (or resistances) when designing encounters and make sure to include things that can still hurt them most of the time, but make sure to have the occasional fight where they get to be invincible and steamroll the enemy.
