Hi! I currently have a living dex and quite a few competitive Pokémon in gen 7. My question is: are there good reasons not to transfer everything in Home and Sword? Thanks in advance for any reply :)
Well, there are four significant reasons why transferring to Home can cause issues. Some may be applicable to you, others may not. Just weigh what works best for your situation.
1. Projects. Perhaps the the largest problem is if you like working on older Pokemon projects. You need resources to breed/train/use most Pokemon, and if you don't have access to them it can severely hamper your ability to work on those Pokemon. Imagine trying to breed perfect Pokemon w/o your various Dittos, or do Battle Tree runs with random wild Pokemon - it won't be fun. Similarly, since Dexit means lots of Pokemon cannot be used in Gen 8, you'll be unable to do anything with those Pokemon or use them in projects, despite them being in Home.
2. Transfer Exclusive Moves. Gen 8 saw the deletion of lots of moves, but it also saw the removal of lots of moves from many learnsets - meaning these moves became transfer exclusive. Perhaps the most notorious one is Toxic, as most Pokemon could learn it prior to Gen 8, and it was a competitive staple. However, in Gen 8, very few Pokemon can actually learn it. If you ever want to change the your mon's moveset, you might not be able to access the necessary moves - despite your Pokemon having been able to learn them in the past.
3. Cost. Home is straight up more expensive than Bank. You'll need to buy Home Prem for both the extra space (if you have lots of mons, which it sounds like you do) and to be able to transfer in the first place. You'll also have to buy Bank access (in order to transfer). I'm not sure about regional price differences, but Home costs ~3x as much as Bank in the US. Coupled with the Switch's already high cost for just about everything (increased game cost, hardware costs, having to pay for Switch Online, etc.) and it really does add up. Your financial situation might prohibit your ability to purchase everything - especially with yearly costs. And, it would really suck if all your Pokemon got deleted beacuse you couldn't afford to pay for Home for one more year.
4. Copies. When Pokemon are transferred into Home, they get a unique tag that identifies them. This shouldn't impact most people, but if you like to have multiple backup copies of Pokemon, Home will flag them if they have the same unique tag and delete them. It's part of Nintendo's anti-piracy measures - which really just make everything more annoying for everyone. You can bypass this by doing any cloning in earlier Gens and then transferring them (they'll get different tags, despite being the same Pokemon - at least that's how I understood it).
These are the broad issues, but they're probably the most impactful. If none of these impact your needs, then you'll probably be okay in transferring. However, if there are some issues for you, I would recommend cloning if possible. Cloning in Gen 6-7 is relatively easy if you have the hardware, and since WiFi for both Gens is still online it's technically possible to find someone who can do it for you. Having a backup clone is usually a good idea regardless, but when doing permanent transfers it can give you a healthy margin if you have an accident.