It is a common issue I would say. And no, if something sounds good on high end sound gear like monitors or some studio headphones designed for mixing it doesn't necessarily have to sound good on low cost gear (ear buds, cheap headphones, smartphones, laptop speakers etc). This is why it's always good to check how you mix sounds on those cheaper gear or on some monitors/headphones or something that will emulate the sound of mediocre gear.
This is why I always check how something sounds on my laptop speakers and phone speakers and believe me it took me years to actually make my kick drums and bass be audible on this kind of things. Everything sounded okay when I was mixing it on my headphones (I use Beyerdynamic DT 770 PRO for making music) but when I played it on my phone it sounded like shit. This kind of mixing is also used by many producers, for example I heard that Kendrick Lamar's producer mixes everything in mono because if something sounds good in mono then it will sound great on every shitty gear. And you have to take it into considerations that most people that listen to music usually don't have some high end headphones or something like that. They just wanna listen to something on their way to work, at work, in the gym etc.
Also you might say: "But hey, what the hell are you even talking about if you mix everything on headphones?" -> And I would reply that it's the matter of knowing your gear that also makes tremendous difference to your mixes. I know how good music sounds on those headphones that I've been using for the last 7 years and I just trust my ears and my headphones. You may believe it or not but I even don't feel the need to plug my Focusrite Scarlett 2i2 in to make good sounding mix. I just know my gear. For sure if I want to make it sound better I plug the audio interface in and I mix. However, you may believe it or not, but sometimes after checking my mix on a regular sound card and on my laptop speakers I'm able to make some elements of my mix sound better than when that audio interface is plugged in. Or maybe that's just my impression.
So I would say there are a few solutions to this issue and to avoid similar situations in the future:
- Check your mix on some budget gear like cheap headphones etc. and make adjustments accordingly
- Mix in mono or check your mix in mono, there are lots of plugins that emulate mono sound
- Use pink noise when mixing (personally, I'm not a fan of this technique but I heard it helps some producers make their mixes better)
- Learn your gear and know its ins and outs
Very bad advice in my opinion. Sounds very unprofessional and it's not a good customer experience.
It's like you left your car at some garage to fix your brakes, a mechanic repairs them partly, you say to him: "But I wanted to repair them fully" and the mechanic replies: "So use your brakes less or drive slower." I know that the example I brought may be a little bit exaggerated but it doesn't resolve the customer's issue. According to them, the mix has something badly done. And for sure they may be wrong about it but replying like this is far from professional.