Well, it's hard to answer a question when different linguists say the opposite things. Especially, not being a linguist myself. Still, I try.
O. Ponomariv, N. Shumarova and O. Avramenko are right in that forms “Доброго дня!” and “Доброго вечора!” were originally untypical for the Ukrainian language (I mean as a separate greeting, not as part of “[бажаю] доброго дня/вечора комусь”). At least, they're almost never used in the Ukrainian literature until the second half of the XX century.
Still, in modern literature that forms are widely used. M. Stelmakh (1951), O. Chornohuz (1977), A. Dnistrovyi (1980), R. Sambuk (1981), E. Rostovtsev (1985), V. Drozd (1989), Ye. Hutsalo, M. Medunytsia (1990), V. Savchenko, Yu. Vynnychuk (1999), V. Nestayko, O. Skrypnyk, (2001), M. Mednikova (2002), N. Tysovska (2004), O. Dermanskyi, N. Sniadanko (2006), L. Denysenko (2007), A. Chekh (2008), V. Lys, Yu. Soroka, O. Volkov (2009), L. Dashvar, M. Riapolova (2010), M. Brynykh, T. Malyarchuk (2012), V. Slapchuk (2013), N. Humeniuk (2014), S. Talan (2015) use them — and the closer to the present time the more they're used. Please note M. Stelmakh, who is not only the first in this list by time, but also one of the major Ukrainian writers. BTW, it's even used within a book name: “Доброго дня, Ярино!” by M. Ishchenko (1960). The short story “Скажи людині: ‘Доброго дня!’” (allegedly) by the other major writer V. Sukhomlynskyi was recommended by Ministry of Education and Science of Ukraine to be used in kindergartens and elementary schools to teach children (courtesy and etc) and is present in several manuals for children's teachers — therefore children are, in fact, taught to say “Доброго дня!”.
So, on the one hand these linguists are right that Ukrainians didn't use such greeting forms originally. On the other hand, they are widely used today. Therefore it's only your decision whether you are going to be a linguistic purist or no.
My personal opinion is: Despite all my respect to O. Ponomariv and others, I won't follow to concretely this their advice. Language is how people speak, not how linguists think people should speak. Yes, during the last decades (or centuries) the Ukrainian language was strongly influenced by Russian and sometimes other languages, and some of its peculiarities received due to that are ugly and should be rolled back. But “Доброго дня!” and “Доброго вечора!” seem to be neither ugly, nor a result external influence. They look like results of natural unification between explicit wish (“доброго дня/вечора вам”) and traditional greeting (“добрий день/вечір”). Thus I don't see any reasons to “protect” language from such changes. Unlike, for example, a trend to use “вірогідний” (reliable) instead of “імовірний” (probable) due to similarity between the former and the Russian “вероятный” (probable).
Timeline of “Добрий ранок!” is almost the same as of “Доброго дня!” and “Доброго вечора!” (i.e. second half of XX century). Still, my personal opinion is not so permissive for it: it neither appears to be an intermediate point between an explicit wish and traditional greeting, nor sounds naturally for my ear (I suspect possible influence of a Russian) — but that's only my opinion.
P. S.: The answer, probably, won't be full if I don't provide proves that “Доброго ранку!”, “Добри(й )день!” and “Добри(й )вечір!” are found in the Ukrainian literature before the second half of the XX century. Here they are:
- — Добрий день, Когуте, ти, співуча птице! А Когутик каже: — Добрий день, Лисице! //I. Franko «Лисиця сповідниця» (1893).
- І як учорашній козак став коло віконечка й сказав їй «добрий вечір», Галя на одвіт йому ледве вимовила свого «доброго вечора». //M. Vovchok «Дев'ять братів і десята сестриця Галя» (1862).
- — Добривечір у хату! — привітали всі москалиху, що стояла посеред хати з ключами в руках. — Або доброго ранку, — одказала вона. //P. Myrnyi «Хіба ревуть воли, як ясла повні?» (1875).