How reliable are HWs in terms of holding funds for long term (maybe 5-6 years into the future)? Like HDDs/thumb drives failing to start or get corrupted, do HWs have any similar issues???
Trezor One is the only device between Ledger and Trezor which was released over 5 years ago in 2014. Trezor T was 2018, Legder Nano S was 2016, and Nano X was 2019. The devices I have used from both companies are still going strong after a variety of times between 2 and 5 years, but there have been some reports of devices failing over time. This will be especially true if you keep them in less than ideal environments (too hot, too cold, too humid, etc.)
I know that having the seed key will help me in restoring my funds in case my HW is lost or damaged. But still I'm a bit skeptical about this cuz I've never used HWs before.
Nothing to be skeptical about. Ledger and Trezor both use the BIP39 standard for seed phrases. I have no concerns with my hardware wallets being lost/stolen/dying on me, exactly because I have my seed phrase backed up somewhere secure. Your coins are never actually
on the hardware device - the hardware device simply gives you a safe and secure way to interact with your private keys, the same private keys which can be restored using your seed phrase in any wallet.
Would you recommend having a backup of the wallets in an offline device in case I hold some good amount of BTC stash in an airgapped PC??
You can have a back up wallet if you like, but it is not necessary. As long as you have your seed phrase backed up securely, that is all you need to restore access to your coins. Having the same seed imported in to multiple wallets can also lessen your security, much like having storing the same data on three different flash drives increases the chance that someone will stumble across one of them.