Reform and Conservative conversion has been a problem for many years, in Eretz Yisroel and abroad.
For years, the Lubavitcher Rebbe zy”a campaigned for Giyur Kehalacha to be the only recognized form of Giyur in EY, and Lubavitcher Rabbonim took to the radio waves and newspapers the world over decrying non Halachic conversions.
Sadly, some Chareidishe elements in EY opposed those efforts, leading to the problems with the Russians today, that many of them who weren’t Jewish were allowed to immigrate to EY, and subsequently married Jewish Israelis.
In my work as a Lubavitcher traveling the world on Shlichus, too often do I come across reform or conservative converts who have been accepted as part of their local Jewish communities, and it’s heart breaking to tell them that they aren’t considered Jewish according to Halacha. (Not always is it appropriate or necessary to inform them…)
Today, we have an (arguably) even bigger problem than reform and Conservative – the so called “Open Orthodox”, who’s converts can proudly claim to have undergone an orthodox conversion, while in all likelihood (I’m no expert, but I imagine in almost all cases) aren’t Halachically Jewish.
Also, we won’t necessarily know when Moshiach comes if someone is now a Mamzer, as the Rambam writes:
ואינו בא לא לטהר הטמא ולא לטמא הטהור, ולא לפסול אנשים שהם בחזקת כשרות ולא להכשיר מי שהוחזקו פסולין אלא לשום שלום בעולם שנאמר והשיב לב אבות על בנים
Melochim, 12:2.