#and if this is for an NPC - i once again wouldn't apply the hybrid genetics as the reason they have chronic health issues.

[theworldbrewery]

Okay,so a tabaxi/elf

Would there be any long-term health concerns or things to consider?

on the level of Official Rules? no. there's nothing official about crossing D&D races having negative downsides other than the usual lifespan differences.

on the level of Roleplay and/or Worldbuilding? do whatever you want -- I'd personally have my player choose 1 or the other for their actual mechanical race and flavor the appearance to give that 'mixed' quality. frankly, the biggest health issue about crossing large genetic gaps is usually that the offspring is often infertile.

If a player wanted to have long-term health issues for their tabaxi-elf PC specifically because of their hybrid nature, I'd draw inspiration from existing species like ligers and mules. But personally, I don't care to depict humanoids as being that 'far apart' genetically -- I treat most humanoids as being as compatible as various human ethnic groups are in real life which is to say -- completely genetically compatible because it's all phenotype differences, not species or subspecies differences.

and so I wouldn't go out of my way to introduce health problems for a player that wasn't deliberately interested in exploring that angle. And if they just wanted to explore, say, a chronically ill or disabled PC, there are plenty of ways that don't involve a mixed-species or "mixed-race" ancestry.

and the mechanics I would build for a PC that wanted mechanical consequences for any malady, illness, or disability hinge so completely on the exact nature of the condition that I don't think I could go into details here on what that might look like.

If this is your table, I'd caution you against treating sapient D&D races as an exercise in genetic compatibility. there are interesting elements at play in this idea, but I just don't think most tables want to spend large amounts of time on their characters' genetic profile so unless you and/or your player have a unique passion for interspecies genetics it's super unlikely to come up in-game without feeling forced and weird.

and if you/your player does have a unique passion for interspecies genetics, far be it from me to instruct someone with more of an interest in how that should play out.