Is it considered bad practice to have a wallet "cloned" on both a smartphone and a desktop PC? I already checked questions as:

Can I use my wallet on different computers?

Can wallets be shared by different machines?

...but what I'm asking regards popular wallets as BitPay and Electrum, that have both the desktop and mobile app, permitting to export and import a wallet. Nothing that takes into account messing with any wallet.dat file.

I'm currently doing this with a BitPay wallet, and it seems it works good. When a transaction occurs (i.e. I receive funds) this is notified on both platforms (I think it's related to how those wallets works; I'm referring to the Bitcore Wallet Service). 

I also read about the notorious quote "not a good idea", but I'm not sure of this refers to different type of wallets or what else...

Any clarification on this is more than welcome.
It is merely a question of security. If your copy of the private key in wallet A (phone) is compromised, you can lose your coins. If your copy of the private key in wallet B (PC) is compromised, you can lose your coins. 

Assuming both copies are secure, you basically just have a backup of your private key.

As for whether or not having two copies like that is secure or "best practice", that depends a lot on how you handle your computers, and there are better experts out there that can answer that.

One thing you cannot do is double spend. If they use the same private key, one wallet is the same as the other, and trying to spend out of both will result in an unconfirmed transaction on one of them.