Why most bitcoin wallet apps don't have Replace-by-fee feature?
Maybe this answer came a bit too late, but lets give it a try.
Let me start by citing A. Narayanan et alter Bitcoin and Cryptocurrency Technologies book:

Since 2013, there has been interest in changing the default policy to
replace-by-fee (RBF) whereby nodes will replace a pending transaction
in their pool if they hear a conflicting transaction which includes a
higher fee. This is the rational behavior for miners, at least in a
short-term sense, as it gives them a better fee.
However, replace-by-fee would make double-spending against zero-confirmation
attacks far easier in practice. Replace-by-fee has therefore attracted
controversy, both in terms of the technical question of whether it is
possible to prevent or deter double-spending in an RBF world, and the
philosophical question of whether Bitcoin should try to support
zero-confirmation as best it can, or abandon it.

So controversy is set between what can be more profitable for miners (i.e accepting transaction with higher fees), versus what may generate more transactions in the mempool and, moreover, could ease double-spending attacks against zero-confirmation transactions. That's probably why Electrum shows such a warning. Merchants could be unwilling to accept transactions that can be replaced if a greater fee is provided before the transaction has been included in a block.
In my humble opinion, it depends on whether your tool will be used for zero-confirmation transactions or not.
Moreover, if you are concerned about what nodes of the network will do when dealing with such a transaction, here you have a question that talks about the relaying policies of RBF.