Unfortunately Nano's fee-less is a "lie".
Long before Nano was created the XRPL team created the first blockchain (DLT) without PoW or PoS and without incentives (fees) for node operators. This lead to a new problem, spam transactions. If transactions are free nothing stops people from sending coins in circles which puts load on the network and thus costs other people real money. The 2 obvious ways to mitigate this is to either add a fee or make the sender solve a little time-consuming math problem (PoW). the XRPL devs wanted to avoid PoW for all the obvious reasons, but fees have also their downside. mainly it creates an incentive and it suddenly makes every transaction that does not include at lest X amount of fee (more than the processing cost) a "negative" Tx i.e. one that all node operators have a reasonable reason to exclude.
This kind of potentially exclude valid Tx, even if there is plenty of capacity, is bad for the network. Users loose the confidence of knowing that a valid Tx will always be included. They would need to intentionally overpay in fees. The XRPL devs decided to mitigate this by adding a fee but its burnt instead of paid to someone. By burning "money" you proof that your Tx is actually worth something to you. How much you burn is basically irrelevant. The network decided whats the minimum and as long as the network inst at full capacity the minimum guarantees that the Tx will be included by all nodes.
Only if the network reaches max throughput it would priories the Tx that burn more. This way an spammer would need to send thousands of Tx every second AND each one of them must burn more than every other legit Tx is willing to burn, to stop everyone from makings Tx go trough.
Now to Nano. They had basically the same problem but choose PoW instead so they can claim its "fee-less" while in reality you pay the "fee" as PoW (energy+hardware+time). Its so minuscule that most user dont even notice that there is a PoW needed. But its there anyway and it comes with the flaws PoW has. Mainly its wasting resources and it does a objectively worse job at preventing spam.
Remember above I explained that a spammer on the XRPL would need to send thousands of Tx and burn (pay) more than every other valid Tx to block them. So the attacker has to be willing to pay more than all user combined. Nano's PoW means an attacker has to do more PoW than all legit user combined but that does NOT mean he has to pay more. PoW, unlike burning, can be optimized. Special hardware, location (cheap energy) and in general doing PoW at large scale, allays will be way way more efficient than what each legit user does.
If thousand user do a total of 1000 Tx and it costs them 0.1 cent (PoW) combined then a single user doing PoW for 1000Tx can certainly do it for way way less then 0.1 cent.
However if 1000 user do a total of 1000 Tx each burns 0.0001 cent (total 0.1 cent) there is no shortcut. A single user would have to burn exactly the same 0.1 cent no way to optimize.
Another negative effect of PoW is that it is not future proof for low-power device.
Imagine some kind of smart sensor or other IoT device that does Tx. It has some tiny tiny "CPU" that may be able to do the PoW for a Tx in a reasonable time. But what about in 10 years? The PoW per Tx inevitably has to rise over time.
Compare that to burning X times more in fees per Tx 10 years from now - doesn't matter the tiny "CPU" will work exactly the same.
(yes, I know you could offload the PoW to another device yada yada yada but K.I.S.S.)
tl;dr:
Nano made a bad decision to make transaction "fees" be paid in PoW for the "benefit" of calling it fee-less.